%!PS-Adobe-1.0
%%Title: GEM Document
%%Creator: GEM
%%Pages: (atend)
%%BoundingBox: 0 0 611 791 
%%EndComments
% Copyright (C) Digital Research, Inc. 1986-1988. All rights reserved.
systemdict /setpacking known
{/svp currentpacking def true setpacking}if
/gemdict 250 dict def
gemdict begin

/bd{bind def}bind def
/ed{exch def}bind def

% User defined Start of Page procedure:  this operator will be
% executed at the beginning of each page output through GEM
% and is provided to allow user-defined page initialization.
/UserSoP{}bd

% Halftone screen spot function procedure array: this array is indexed
% into as follows: 0 = dot screen, 1 = line screen, 2 = ellipse screen,
% 3 = custom (user-definable) screen.
/ScreenProc[
	{ % Dot screen
		abs exch abs 2 copy add 1 gt{
			1 sub dup mul exch 1 sub dup mul add 1 sub}{
			dup mul exch dup mul add 1 exch sub}ifelse}bind
	{ % Line screen
		pop}bind
	{ % Ellipse screen
		dup 5 mul 8 div mul exch dup mul exch add sqrt 1 exch sub}bind
	{ % Custom screen
		dup mul exch dup mul add sqrt 1 exch sub}bind
	]def

/GEMINISUB{pop pop pop pop pop}bd
/GEMINIDOC{dup userdict exch known{cvx exec}{pop}ifelse
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	statusdict /setpageparams known
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/geminit{np 1 setlinejoin /mpf true def
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/GEMMATSUB{pop pop pop pop gr 72 300 div exch div dup scale gs}bd
/GEMMATINI{/landscape ed /p3 ed /p2 ed /p1 ed
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	exch /prx ed exch dup /ply ed sub 1 add p3 sub 2 div ply
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/gr /grestore load def
/np /newpath load def
/cp /currentpoint load def
/gi /getinterval load def
/lto /lineto load def
/mto /moveto load def
/cto /curveto load def
/clw /currentlinewidth load def
/gs /gsave load def 
/greset{gr gs}bd
/settray{dup 0 lt mf dup 0 ge{tray}{pop}ifelse}bd
/mf{statusdict /manualfeed known
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/tray{statusdict begin true
	product(Dataproducts LZR 1260)eq{not exch dup 2 gt{pop 0}if
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	product(QMS-PS 1200)eq product(Silentwriter)eq or{not exch dup 1 gt
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	product dup(PrintServer 40)eq exch(QMS-PS 2400)eq or{
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		{dup defaultpapertray ge{1 add}if}ifelse setpapertray}if
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	8#214 /icircumflex  8#215 /igrave 8#216 /Adieresis 8#217 /Aring
	8#220 /Eacute 8#221 /ae 8#222 /AE 8#223 /ocircumflex 8#224 /odieresis
	8#225 /ograve 8#226 /ucircumflex 8#227 /ugrave 8#230 /ydieresis
	8#231 /Odieresis 8#232 /Udieresis 8#233 /cent 8#234 /sterling
	8#235 /yen 8#236 /currency 8#237 /florin 8#240 /aacute 8#241 /iacute
	8#242 /oacute 8#243 /uacute 8#244 /ntilde 8#245 /Ntilde
	8#246 /ordfeminine 8#247 /ordmasculine 8#250 /questiondown
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	8#300 /quotedblbase 8#301 /ellipsis 8#302 /perthousand 8#303 /bullet
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	8#310 /Acircumflex 8#311 /Egrave 8#312 /Ecircumflex 8#313 /Edieresis
	8#314 /Igrave 8#315 /Iacute 8#316 /Icircumflex 8#317 /Idieresis
	8#320 /Ograve 8#321 /Oacute 8#322 /Ocircumflex 8#323 /Scaron
	8#324 /scaron 8#325 /Ugrave 8#326 /Uacute 8#327 /Ucircumflex
	8#330 /Ydieresis 8#331 /germandbls 8#332 /Zcaron 8#333 /zcaron
	8#334 /fraction 8#335 /space 8#336 /space 8#337 /space 8#340 /grave
	8#341 /acute 8#342 /circumflex 8#343 /tilde 8#344 /dieresis
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/encfont{gemenvec exch fonts exch get dup encstr cvs length 1 sub encstr
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/dpath{mto{lto}repeat}bd
/path{np dpath}bd
/addpath{lto{lto}repeat}bd
/rxy{.25 sub round .25 add}bd
/rpt{transform rxy exch rxy exch itransform}bd
/fa{np rpt mto 3{rpt lto}repeat}bd
/circle{np 0 0 1 0 360 arc}bd
/dot{gs np 2 copy mto lto 1 setlinecap stroke gr}bd
/rend{gs 1 setlinecap np mto cp 0.1 add lto stroke gr}bd
/rl{gs 1 setlinecap stroke gr}bd
/vl{dup /st ed dup apath exch get tx sub dup mul
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/doarrow{/rot ed /ty ed /tx ed 6 array currentmatrix
	tx ty translate clw 4 lt{4}{clw}ifelse dup scale rot rotate np
	0 0 mto -3 1.5 lto -3 -1.5 lto fill setmatrix}bd
/arpath{np apath 0 get apath 1 get mto 2 2 points 1 sub
	{dup apath exch get exch 1 add apath exch get lto}for}bd
/arrowline{/apath ed /lend ed /lbeg ed /len clw 3 mul def
	/points apath length def lbeg
	{apath 0 get dup /tx ed /x1 ed apath 1 get dup
		/ty ed /y1 ed true 2 2 points 1 sub
		{vl len ge{pop false exit}if}for
		{/lbeg false def /lend false def}
		{apath 2 apath st points st sub gi putinterval /r1 y1
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			sub put /points points st 2 sub sub def}ifelse}if
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		true points 4 sub -2 0
		{vl len ge{pop false exit}if}for
		{/lbeg false def /lend false def}
		{/r2 y2 apath st 1 add get sub x2 apath st get sub
			atan def /st st 2 add def apath st x2 r2 cos len mul
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			/points st 2 add def}ifelse}if
	lbeg{x1 y1 r1 doarrow}if lend{x2 y2 r2 doarrow}if arpath}bd
/ac{6 array currentmatrix xt yt translate xs ys scale}bd
/shorten{dup mul exch dup mul add sqrt clw 150 mul exch div}bd
/xang{dup 360 ge{360}{0}ifelse exch dup sin xs mul exch cos ys mul atan
	dup 360 lt{add}{exch pop}ifelse}bd
/arrowarc{/eang ed /bang ed /ys ed /xs ed /yt ed
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	lend{x2 y2 cp y2 exch sub exch x2 exch sub atan doarrow}if
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/rbox{/ury ed /urx ed /lly ed /llx ed urx llx sub
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	ury llx lly radius arcto 4{pop}repeat llx lly urx lly radius arcto
	4{pop}repeat urx lly urx ury radius arcto 4{pop}repeat urx ury llx ury
	radius arcto 4{pop}repeat}bd
/marker{1 sub mdef exch get /mproc ed 32 div /msize ed
	{gs np translate msize dup scale mproc stroke gr}repeat}bd
/mdef[{0 0 mto 1 0 lto 1 1 lto 0 1 lto closepath}bind
	{-16 0 mto 16 0 lto 0 -16 mto 0 16 lto}bind
	{0 -16 mto 0 16 lto 13.9 8 mto -13.9 -8 lto 13.9 -8 mto
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	{16 16 mto -16 16 lto -16 -16 lto 16 -16 lto closepath}bind
	{16 16 mto -16 -16 lto -16 16 mto 16 -16 lto}bind
	{16 0 mto 0 16 lto -16 0 lto 0 -16 lto closepath}bind]def
/bon{2 mul exch dup 3 1 roll 8 idiv add pstr exch get
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/bpsf{1 add 8 mul cvi exch 1 add 8 mul cvi exch bon
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/frs{72 0 rmat defaultmatrix dtransform dup mul exch dup mul add sqrt}bd
/sus{/m cmat currentmatrix def /sm 32 dup smat scale def sm m m concatmatrix
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/decode{/patstring patlen string def /bonestr 1 string def
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/psc{dup length /plen ed exch -1 1
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/gtext{gs /msg ed /ty ed /tx ed tx ty translate trotate
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/etext{gs translate trotate rotate /tx 0 def /ty 0 def
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/jtext{/msg ed /sps ed /dx ed /ty ed /tx ed
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/fet{gs translate /tx 0 def /ty 0 def
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/fjt{/msg ed /sps ed /dx ed /ty ed
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/horz[{/tx 0 def}bind
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/extents{(_)bbox pop pop msg stringwidth pop (])bbox 3{exch pop}repeat}bd
/bbox{np 0 0 mto false charpath flattenpath pathbbox np}bd
/vert[{/ty 0 def}bind
	{extents -2 div /ty ed pop pop pop}bind
	{extents neg /ty ed pop pop pop}bind
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/jhorz[{/tx 0 def}bind
	{/tx dx -2 div def}bind
	{/tx dx neg def}bind]def
/symindex 12 def
/CR{/ah 0 def}bd
/LF{0 -50 translate}bd
/atext{gs np ah av mto
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end
/vpdict 1 dict def
systemdict /setpacking known{svp setpacking}if
%%EndProlog
gsave
currentdict /gemdict known not {
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newpath 72 686 moveto
(  printer.  Pre-download the preamble or include it with)show
newpath 72 672 moveto(  your print job.)show
newpath 72 658 moveto(This print job has been aborted.)show
showpage stop}if gemdict begin
684 864 0 1 /letter GEMINIDOC geminit
1 2549 2550 3300 false GEMMATINI
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%Begin page
UserSoP
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sf
300 1743 473 3 (Status of this Memo)fjt
/tface 8 def
8 encfont
sf
300 1655 1950 12 (This document describes the Network Time Protocol \(NTP\), specifies its formal structure and)fjt
(o)14 (t)25 ( )20 (s)40 (m)20 (s)14 (i)26 (n)23 (a)26 (h)22 (c)23 (e)40 (m)25 ( )22 (e)26 (h)15 (t)25 ( )20 (s)22 (e)26 (d)15 (i)25 (v)26 (o)17 (r)26 (p)25 ( )29 (P)31 (T)37 (N)25 ( )13 (.)26 (n)25 (o)15 (i)15 (t)23 (a)14 (t)26 (n)23 (e)40 (m)23 (e)15 (l)26 (p)40 (m)14 (i)25 ( )21 (s)14 (t)15 (i)25 ( )18 (r)26 (o)17 (f)25 ( )15 (l)26 (u)17 (f)23 (e)21 (s)26 (u)25 ( )26 (n)26 (o)14 (i)15 (t)23 (a)40 (m)17 (r)26 (o)18 (f)26 (n)15 (i)25 ( )20 (s)23 (e)23 (z)15 (i)17 (r)23 (a)40 (m)40 (m)26 (u)20 (s)0 84 300 1597 fet
300 1539 1788 13 (synchronize time and coordinate time distribution in a large, diverse internet operating at ra)fjt
2088 1539 162 1 (tes from)fjt
300 1481 1867 15 (mundane to lightwave. It uses a returnable-time design in which a distributed subnet of time ser)fjt
2167 1481 83 0 (vers)fjt
300 1422 1859 8 (operating in a self-organizing, hierarchical-master-slave configuration synchronizes local cl)fjt
2159 1422 91 0 (ocks)fjt
300 1364 1834 16 (within the subnet and to national time standards via wire or radio. The servers can also redist)fjt
2134 1364 116 0 (ribute)fjt
300 1306 1237 8 (reference time via local routing algorithms and time daemons.)fjt
300 1219 1845 12 (This is an Internet Standard Recommended Protocol. Distribution of this memo is unlimited.)fjt
300 1131 1874 8 (Keywords: network clock synchronization, standard time distribution, fault-tolerant architect)fjt
2174 1131 76 0 (ure,)fjt
300 1073 1782 7 (maximum-likelihood estimation, disciplined oscillator, internet protocol, formal specifica)fjt
2082 1073 90 0 (tion.)fjt
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(\))32 (2)16 ( )36 (n)35 (o)16 (i)33 (s)22 (r)32 (e)36 (V)19 (\()16 ( )16 (l)36 (o)32 (c)36 (o)19 (t)35 (o)23 (r)39 (P)16 ( )32 (e)52 (m)16 (i)34 (T)16 ( )32 (k)23 (r)35 (o)46 (w)19 (t)32 (e)42 (N)0 33 804 2306 fet
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300 3034 532 2 (Network Working Group)fjt
1954 3034 296 2 (David L. Mills)fjt
300 2976 651 3 (Request for Comments: 1119)fjt
1752 2976 498 2 (University of Delaware)fjt
300 2918 712 2 (Obsoletes: RFC-1059, RFC-958)fjt
1882 2918 369 1 (September 1989)fjt
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300 2729 38 0 (1.)fjt
450 2729 259 1 ( Introduction)fjt
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450 2671 415 2 ( Related Technology)fjt
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2225 2671 25 0 (2)fjt
300 2613 38 0 (2.)fjt
450 2613 420 2 ( System Architecture)fjt
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300 2554 75 0 (2.1.)fjt
450 2554 470 2 ( Implementation Model)fjt
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300 2496 75 0 (2.2.)fjt
450 2496 498 2 ( Network Configurations)fjt
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2225 2496 25 0 (5)fjt
300 2438 75 0 (2.3.)fjt
450 2438 413 3 ( The NTP Timescale)fjt
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300 2380 75 0 (2.4.)fjt
450 2380 389 3 ( The NTP Calendar)fjt
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2225 2380 25 0 (8)fjt
300 2321 75 0 (2.5.)fjt
450 2321 725 4 ( Time and Frequency Dissemination)fjt
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300 2263 38 0 (3.)fjt
450 2263 486 3 ( Network Time Protocol)fjt
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300 2205 75 0 (3.1.)fjt
450 2205 282 2 ( Data Formats)fjt
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450 2147 632 4 ( State Variables and Parameters)fjt
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2200 2147 50 0 (12)fjt
300 2088 113 0 (3.2.1.)fjt
450 2088 401 2 ( Common Variables)fjt
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450 2030 362 2 ( System Variables)fjt
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300 1972 113 0 (3.2.3.)fjt
450 1972 304 2 ( Peer Variables)fjt
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300 1914 113 0 (3.2.4.)fjt
450 1914 348 2 ( Packet Variables)fjt
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2200 1914 50 0 (17)fjt
300 1855 113 0 (3.2.5.)fjt
450 1855 454 3 ( Clock Filter Variables)fjt
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450 1797 233 1 ( Parameters)fjt
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450 1739 414 3 ( Modes of Operation)fjt
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450 1622 406 2 ( Transmit Procedure)fjt
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450 1564 387 2 ( Receive Procedure)fjt
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450 1506 359 2 ( Packet Procedure)fjt
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450 1448 509 2 ( Primary-clock procedure)fjt
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450 1389 492 2 ( Clock-update procedure)fjt
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450 1331 481 2 ( Initialization Procedure)fjt
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450 1156 452 3 ( Access Control Issues)fjt
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450 1098 705 4 ( Filtering and Selection Algorithms)fjt
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300 1040 75 0 (4.1.)fjt
450 1040 456 2 ( Clock-filter procedure)fjt
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450 982 536 2 ( Clock-selection procedure)fjt
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450 923 277 2 ( Local Clocks)fjt
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300 865 75 0 (5.1.)fjt
450 865 417 2 ( Standard Oscillators)fjt
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450 807 428 2 ( Mathematical Model)fjt
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300 749 75 0 (5.3.)fjt
450 749 511 2 ( Fuzzball Implementation)fjt
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300 690 75 0 (5.4.)fjt
450 690 577 3 ( Uniform Phase Adjustments)fjt
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450 632 652 3 ( Nonuniform Phase Adjustments)fjt
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300 574 75 0 (5.6.)fjt
450 574 564 4 ( Maintaining Date and Time)fjt
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2200 574 50 0 (40)fjt
300 516 38 0 (6.)fjt
450 516 386 1 ( Acknowledgments)fjt
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300 399 38 0 (8.)fjt
450 399 876 8 ( Appendix A. NTP Data Format - Version 2)fjt
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2200 399 50 0 (45)fjt
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300 2841 38 0 (9.)fjt
450 2841 745 5 ( Appendix B. NTP Control Messages)fjt
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300 2782 75 0 (9.1.)fjt
450 2782 614 4 ( NTP Control Message Format)fjt
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2200 2782 50 0 (49)fjt
300 2724 75 0 (9.2.)fjt
450 2724 279 2 ( Status Words)fjt
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300 2666 113 0 (9.2.1.)fjt
450 2666 419 3 ( System Status Word)fjt
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2200 2666 50 0 (51)fjt
300 2608 113 0 (9.2.2.)fjt
450 2608 361 3 ( Peer Status Word)fjt
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2200 2608 50 0 (52)fjt
300 2549 113 0 (9.2.3.)fjt
450 2549 392 3 ( Clock Status Word)fjt
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300 2491 113 0 (9.2.4.)fjt
450 2491 378 3 ( Error Status Word)fjt
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300 2433 75 0 (9.3.)fjt
450 2433 240 1 ( Commands)fjt
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300 2375 63 0 (10.)fjt
450 2375 710 4 ( Appendix C. Authentication Issues)fjt
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300 2316 100 0 (10.1.)fjt
450 2316 660 3 ( NTP Authentication Mechanism)fjt
1125 2316 1050 28 ( . . . . . . . . . . . . . . . . . . . . . . . . . . . .)fjt
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300 2258 100 0 (10.2.)fjt
450 2258 649 3 ( NTP Authentication Procedures)fjt
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300 2200 63 0 (11.)fjt
450 2200 1012 6 ( Appendix D. Differences from Previous Versions.)fjt
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300 2113 343 2 (List of Figures)fjt
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300 2025 650 3 (Figure 1. Implementation Model)fjt
975 2025 1238 33 ( . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .)fjt
2225 2025 25 0 (4)fjt
300 1967 774 5 (Figure 2. Calculating Delay and Offset)fjt
1088 1967 1088 29 ( . . . . . . . . . . . . . . . . . . . . . . . . . . . . .)fjt
2200 1967 50 0 (25)fjt
300 1909 688 4 (Figure 3. Phase-Lock Loop Model)fjt
1013 1909 1163 31 ( . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .)fjt
2200 1909 50 0 (35)fjt
300 1850 509 3 (Figure 4. Clock Registers)fjt
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2200 1850 50 0 (37)fjt
300 1792 629 4 (Figure 5. NTP Message Header)fjt
938 1792 1238 33 ( . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .)fjt
2200 1792 50 0 (45)fjt
300 1734 794 5 (Figure 6. NTP Control Message Header)fjt
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300 1676 616 4 (Figure 7. Status Word Formats)fjt
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300 1617 622 3 (Figure 8. Authenticator Format)fjt
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300 1530 321 2 (List of Tables)fjt
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300 1442 807 5 (Table 1. Dates of Leap-Second Insertion)fjt
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300 1384 525 3 (Table 2. System Variables)fjt
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300 1326 467 3 (Table 3. Peer Variables)fjt
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300 1268 511 3 (Table 4. Packet Variables)fjt
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300 1209 396 2 (Table 5. Parameters)fjt
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300 1151 563 4 (Table 6. Modes and Actions)fjt
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300 1093 942 5 (Table 7. Characteristics of Standard Oscillators)fjt
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300 1035 528 3 (Table 8. Clock Parameters)fjt
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300 2751 1950 13 (This document constitutes a formal specification of the Network Time Protocol \(NTP\), which is)fjt
300 2692 1903 15 (used to synchronize timekeeping among a set of distributed time servers and clients. It defines t)fjt
2203 2692 47 0 (he)fjt
300 2634 1895 13 (architectures, algorithms, entities and protocols used by NTP and is intended primarily for i)fjt
2195 2634 39 0 (m)fjt
2234 2634 17 0 (-)fjt
300 2576 1934 10 (plementors. A companion document [44] summarizes the requirements, analytical models, algo)fjt
2234 2576 17 0 (-)fjt
300 2518 1925 12 (rithmic analysis and performance under typical Internet conditions. NTP was first described i)fjt
2225 2518 25 0 (n)fjt
300 2459 1950 15 (RFC-958 [30], but has evolved in significant ways, culminating in the most recent NTP Version 1)fjt
300 2401 1889 16 (described in RFC-1059 [42]. It is built on the Internet Protocol \(IP\) [14] and User Datagram Proto)fjt
2189 2401 61 0 (col)fjt
300 2343 1914 12 (\(UDP\) [9], which provide a connectionless transport mechanism; however, it is readily adaptab)fjt
2214 2343 36 0 (le)fjt
300 2285 1950 15 (to other protocol suites. NTP is evolved from the Time Protocol [19] and the ICMP Timestamp)fjt
300 2226 1950 13 (message [15], but is specifically designed to maintain accuracy and robustness, even when used)fjt
300 2168 1818 11 (over typical Internet paths involving multiple gateways, highly dispersive delays and unr)fjt
2118 2168 132 0 (eliable)fjt
300 2110 93 0 (nets.)fjt
300 2017 1950 11 (The service environment consists of the implementation model, service model and timescale)fjt
300 1959 1876 13 (described in Section 2. The implementation model is based on a multiple-process operating sys)fjt
2176 1959 75 0 (tem)fjt
300 1901 1888 14 (architecture, although other architectures could be used as well. The service model is based o)fjt
2188 1901 62 1 (n a)fjt
300 1842 1914 13 (returnable-time design which depends only on measured clock offsets, but does not require reliab)fjt
2214 1842 36 0 (le)fjt
300 1784 1928 8 (message delivery. The synchronization subnet uses a self-organizing, hierarchical-master-slav)fjt
2228 1784 22 0 (e)fjt
300 1726 1950 10 (configuration, with synchronization paths determined by a minimum-weight spanning tree. While)fjt
300 1668 1854 13 (multiple masters \(primary servers\) may exist, there is no requirement for an election protocol)fjt
2154 1668 13 0 (.)fjt
300 1575 1950 15 (NTP itself is described in Section 3. It provides the protocol mechanisms to synchronize time in)fjt
300 1517 1911 14 (principle to precisions in the order of nanoseconds while preserving a non-ambiguous date well in)fjt
2211 1517 39 0 (to)fjt
300 1458 1889 13 (the next century. The protocol includes provisions to specify the characteristics and estimate )fjt
2189 1458 61 0 (the)fjt
300 1400 1950 17 (error of the local clock and the time server to which it may be synchronized. It also includes)fjt
300 1342 1834 11 (provisions for operation with a number of mutually suspicious, hierarchically distributed pr)fjt
2134 1342 116 0 (imary)fjt
300 1284 770 5 (reference sources such as radio clocks.)fjt
300 1191 1845 11 (Section 4 describes algorithms useful for deglitching and smoothing clock-offset samples coll)fjt
2145 1191 105 0 (ected)fjt
300 1132 1950 15 (on a continuous basis. These algorithms evolved from with suggested in [28], were refined as the)fjt
300 1074 1808 12 (results of experiments described in [29] and further evolved under typical operating conditio)fjt
2108 1074 143 1 (ns over)fjt
300 1016 1820 14 (the last three years. In addition, as the result of experience in operating multiple-server s)fjt
2120 1016 130 0 (ubnets)fjt
300 958 1950 15 (including radio-synchronized clocks at several sites in the U.S. and with clients in the U.S. and)fjt
300 899 1812 12 (Europe, reliable algorithms for selecting good clocks from a population possibly including )fjt
2112 899 139 0 (broken)fjt
300 841 1145 9 (ones have been developed and are described in Section 4.)fjt
300 749 1950 13 (The accuracies achievable by NTP depend strongly on the precision of the local-clock hardware)fjt
300 690 1928 14 (and stringent control of device and process latencies. Provisions must be included to adjust th)fjt
2228 690 22 0 (e)fjt
300 632 1925 13 (software logical-clock time and frequency in response to corrections produced by NTP. Section )fjt
2225 632 25 0 (5)fjt
300 574 1896 13 (describes a local-clock design evolved from the Fuzzball implementation described in [21] and [4)fjt
2196 574 54 0 (3].)fjt
300 516 1950 9 (This design includes offset-slewing, drift-compensation and deglitching mechanisms capable of)fjt
300 457 1950 13 (accuracies in the order of a millisecond, even after extended periods when synchronization to)fjt
300 399 798 5 (primary reference sources has been lost.)fjt
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300 2841 1950 14 (Details specific to NTP packet formats used with the Internet Protocol \(IP\) and User Datagram)fjt
300 2782 1950 14 (Protocol \(UDP\) are presented in Appendix A, while details of a suggested auxiliary NTP Control)fjt
300 2724 1950 11 (Message, which may be used when comprehensive network-monitoring facilities are not available,)fjt
300 2666 1874 12 (are presented in Appendix B. Appendix C contains specification and implementation details o)fjt
2174 2666 76 1 (f an)fjt
300 2608 1925 12 (optional authentication mechanism which can be used to control access and prevent unauthorize)fjt
2225 2608 25 0 (d)fjt
300 2549 1925 14 (data modification. Appendix D contains a listing of differences between Version 2 of NTP an)fjt
2225 2549 25 0 (d)fjt
300 2491 362 1 (previous versions.)fjt
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300 2398 83 0 (1.1.)fjt
(y)29 (g)30 (o)12 (l)30 (o)29 (n)30 (h)27 (c)26 (e)26 (T)13 ( )30 (d)26 (e)16 (t)27 (a)12 (l)27 (e)35 (R)13 ( )0 19 383 2398 fet
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300 2308 1889 15 (Other mechanisms have been specified in the Internet protocol suite to record and transmit the ti)fjt
2189 2308 61 0 (me)fjt
300 2250 1950 14 (at which an event takes place, including the Daytime protocol [18], Time Protocol [19], ICMP)fjt
300 2191 1950 12 (Timestamp message [15] and IP Timestamp option [13]. Experimental results on measured times)fjt
300 2133 1851 15 (and roundtrip delays in the Internet are discussed in [20], [29], [41] and [42]. Other synchroniz)fjt
2151 2133 100 0 (ation)fjt
300 2075 1828 15 (algorithms are discussed in [4], [22], [23], [24], [25], [27], [28], [29], [30], [31], [33], [35])fjt
2128 2075 122 1 (, [38],)fjt
300 2017 1896 17 ([39], [40], [42] and [44], while protocols based on them are described in [11], [12], [21], [26], [3)fjt
2196 2017 54 0 (0],)fjt
300 1958 1950 14 ([35], [42] and [44]. NTP uses techniques evolved from them and both linear-systems and agreement)fjt
300 1900 1950 10 (methodologies. Linear methods for digital telephone network synchronization are summarized in)fjt
300 1842 1603 10 ([6], while agreement methods for clock synchronization are summarized in [25].)fjt
300 1749 1873 10 (The Fuzzball routing protocol [21], sometimes called Hellospeak, incorporates time synchroniza)fjt
2173 1749 78 0 (tion)fjt
300 1691 1821 13 (directly into the routing-protocol design. One or more processes synchronize to an external ref)fjt
2121 1691 130 0 (erence)fjt
300 1633 1950 15 (source, such as a radio clock or NTP daemon, and the routing algorithm constructs a minimum-)fjt
300 1575 1860 14 (weight spanning tree rooted on these processes. The clock offsets are then distributed along the)fjt
2160 1575 90 1 ( arcs)fjt
300 1516 1900 16 (of the spanning tree to all processes in the system and the various process clocks corrected usi)fjt
2200 1516 50 0 (ng)fjt
300 1458 1950 17 (the procedure described in Section 5 of this document. While it can be seen that the design of)fjt
300 1400 1828 13 (Hellospeak strongly influenced the design of NTP, Hellospeak itself is not an Internet protoc)fjt
2128 1400 122 1 (ol and)fjt
300 1342 1042 7 (is unsuited for use outside its local-net environment.)fjt
300 1249 623 5 (The Unix 4.3bsd time daemon )fjt
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923 1249 111 0 (timed)fjt
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1033 1249 1217 10 ( [26] uses a single master-time daemon to measure offsets of)fjt
300 1191 1809 17 (a number of slave hosts and send periodic corrections to them. In this model the master is dete)fjt
2109 1191 141 0 (rmined)fjt
300 1132 1909 15 (using an election algorithm [31] designed to avoid situations where either no master is elected )fjt
2209 1132 42 0 (or)fjt
300 1074 1911 15 (more than one master is elected. The election process requires a broadcast capability, which is n)fjt
2211 1074 39 0 (ot)fjt
300 1016 1854 14 (a ubiquitous feature of the Internet. While this model has been extended to support hierarc)fjt
2154 1016 97 0 (hical)fjt
300 958 1950 17 (configurations in which a slave on one network serves as a master on the other [35], the model)fjt
300 899 1879 12 (requires handcrafted configuration tables in order to establish the hierarchy and avoid loops)fjt
2179 899 71 1 (. In)fjt
300 841 1781 12 (addition to the burdensome, but presumably infrequent, overheads of the election process, t)fjt
2081 841 170 1 (he offset)fjt
300 783 1706 10 (measurement/correction process requires twice as many messages as NTP per update.)fjt
300 690 1912 16 (A scheme with features similar to NTP is described in [39]. This scheme is intended for multi-serv)fjt
2212 690 39 0 (er)fjt
300 632 1853 15 (LANs where each of a set of possibly many time servers determines its local-time offset relati)fjt
2153 632 97 1 (ve to)fjt
300 574 1950 14 (each of the other servers in the set using periodic timestamped messages, then determines the)fjt
300 516 1950 11 (local-clock correction using the Fault-Tolerant Average \(FTA\) algorithm of [24]. The FTA)fjt
300 457 812 7 (algorithm, which is useful where up to )fjt
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1112 457 22 0 (k)fjt
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1134 457 1095 10 ( servers may be faulty, sorts the offsets, discards the )fjt
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300 399 1950 17 (highest and lowest ones and averages the rest. The scheme, as described in [39], is most suitable to)fjt
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300 2841 1950 12 (LAN environments which support broadcast and would result in unacceptable overhead in an)fjt
300 2782 1879 14 (internet environment. In addition, for reasons given in Section 4 of this paper, the statist)fjt
2179 2782 72 0 (ical)fjt
300 2724 1786 15 (properties of the FTA algorithm are not likely to be optimal in an internet environment wit)fjt
2086 2724 164 1 (h highly)fjt
300 2666 354 1 (dispersive delays.)fjt
300 2577 1912 17 (A good deal of research has gone into the issue of maintaining accurate time in a community whe)fjt
2212 2577 39 0 (re)fjt
300 2518 686 6 (some clocks cannot be trusted. A )fjt
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986 2518 218 0 (truechimer)fjt
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1204 2518 1047 9 ( is a clock that maintains timekeeping accuracy to a)fjt
300 2460 1023 7 (previously published \(and trusted\) standard, while a )fjt
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1323 2460 207 0 (falseticker)fjt
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1529 2460 721 7 ( is a clock that does not. Determining)fjt
300 2402 1749 14 (whether a particular clock is a truechimer or falseticker is an interesting abstract problem )fjt
2049 2402 201 1 (which can)fjt
300 2344 1341 9 (be attacked using agreement methods summarized in [25] and [38].)fjt
300 2252 1903 15 (A convergence function operates upon the offsets between the clocks in a system to increase t)fjt
2203 2252 47 0 (he)fjt
300 2194 1950 13 (accuracy by reducing or eliminating errors caused by falsetickers. There are two classes of)fjt
300 2136 1823 8 (convergence functions, those involving interactive-convergence algorithms and those inv)fjt
2123 2136 128 0 (olving)fjt
300 2078 1698 5 (interactive-consistency algorithms. Interactive-convergence algorithms use statistic)fjt
1998 2078 253 1 (al clustering)fjt
300 2019 1928 14 (techniques such as the fault-tolerant average algorithm of [23], the CNV algorithm of [24], th)fjt
2228 2019 22 0 (e)fjt
300 1961 1807 11 (majority-subset algorithm of [28], the non-Byzantine algorithm of [40], the egocentric algor)fjt
2107 1961 143 1 (ithm of)fjt
300 1903 1076 9 ([33] and the algorithms in Section 4 of this document.)fjt
300 1812 1911 10 (Interactive-consistency algorithms are designed to detect faulty clock processes which mig)fjt
2211 1812 39 0 (ht)fjt
300 1753 1686 11 (indicate grossly inconsistent offsets in successive readings or to different readers. The)fjt
1986 1753 265 1 (se algorithms)fjt
300 1695 1873 12 (use an agreement protocol involving successive rounds of readings, possibly relayed and poss)fjt
2173 1695 78 0 (ibly)fjt
300 1637 1820 13 (augmented by digital signatures. Examples include the fireworks algorithm of [23] and the op)fjt
2120 1637 130 0 (timum)fjt
300 1579 1830 11 (algorithm of [38]. However, these algorithms require large numbers of messages, especially)fjt
2130 1579 121 1 ( when)fjt
300 1520 1900 16 (large numbers of clocks are involved, and are designed to detect faults that have rarely been fou)fjt
2200 1520 50 0 (nd)fjt
300 1462 1845 14 (in the Internet experience. For these reasons they are not considered further in this document)fjt
2145 1462 13 0 (.)fjt
300 1371 1910 15 (In practice it is not possible to determine the truechimers from the falsetickers on other than)fjt
2210 1371 40 1 ( a)fjt
300 1312 1658 10 (statistical basis, especially with hierarchical configurations and a statistically noisy )fjt
1958 1312 292 1 (Internet. Thus,)fjt
300 1254 1831 12 (the approach taken in this document and its predecessors involves mutually coupled oscillato)fjt
2131 1254 119 1 (rs and)fjt
300 1196 1903 11 (maximum-likelihood estimation and selection procedures. From the analytical point of view, t)fjt
2203 1196 47 0 (he)fjt
300 1138 1950 14 (system of distributed NTP peers operates as a set of coupled phase-locked oscillators, with the)fjt
300 1079 1762 14 (update algorithm functioning as a phase detector and the local clock as a disciplined oscill)fjt
2062 1079 189 1 (ator. This)fjt
300 1021 1790 14 (similarity is not accidental, since systems like this have been studied extensively [6], [7] a)fjt
2090 1021 133 1 (nd [8].)fjt
300 930 1879 12 (The particular choice of offset measurement and computation procedure described in Section )fjt
2179 930 72 1 (3 is)fjt
300 872 1870 15 (a variant of the returnable-time system used in some digital telephone networks [6]. The clock f)fjt
2170 872 80 0 (ilter)fjt
300 814 1873 12 (and selection algorithms are designed so that the clock synchronization subnet self-organizes )fjt
2173 814 78 0 (into)fjt
300 755 1950 12 (a hierarchical-master-slave configuration [8]. What makes the NTP model unique is the adaptive)fjt
300 697 1805 12 (configuration, polling, filtering and selection functions which tailor the dynamics of the sy)fjt
2105 697 145 1 (stem to)fjt
300 639 784 4 (fit the ubiquitous Internet environment.)fjt
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300 457 1889 15 (The purpose of NTP is to connect a number of primary reference sources, synchronized to natio)fjt
2189 457 61 0 (nal)fjt
300 399 1950 13 (standards by wire or radio, to widely accessible resources such as backbone gateways. These)fjt
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300 2014 1950 14 (gateways, acting as primary time servers, use NTP between them to cross-check the clocks and)fjt
300 1956 1950 13 (mitigate errors due to equipment or propagation failures. Some number of local-net hosts or)fjt
300 1898 1950 16 (gateways, acting as secondary time servers, run NTP with one or more of the primary servers. In)fjt
300 1840 1848 15 (order to reduce the protocol overhead, the secondary servers distribute time via NTP to the rema)fjt
2148 1840 103 0 (ining)fjt
300 1781 1850 14 (local-net hosts. In the interest of reliability, selected hosts can be equipped with less accurat)fjt
2150 1781 100 1 (e but)fjt
300 1723 1909 16 (less expensive radio clocks and used for backup in case of failure of the primary and/or seconda)fjt
2209 1723 42 0 (ry)fjt
300 1665 937 5 (servers or communication paths between them.)fjt
300 1571 1931 14 (There is no provision for peer discovery or virtual-circuit management in NTP. Data integrity i)fjt
2231 1571 19 0 (s)fjt
300 1513 1934 11 (provided by the IP and UDP checksums. No circuit-management, duplicate-detection or retransmis)fjt
2234 1513 17 0 (-)fjt
300 1454 1950 15 (sion facilities are provided or necessary. The service can operate in a symmetric mode, in which)fjt
300 1396 1826 12 (servers and clients are indistinguishable, yet maintain a small amount of state information)fjt
2126 1396 124 2 (, or in)fjt
300 1338 1889 15 (client/server mode, in which servers need maintain no state other than that contained in the cli)fjt
2189 1338 61 0 (ent)fjt
300 1280 1950 8 (request. A lightweight association-management capability, including dynamic reachability and)fjt
300 1221 1950 12 (variable polling-rate mechanisms, is included only to manage the state information and reduce)fjt
300 1163 1950 14 (resource requirements. Since only a single NTP message format is used, the protocol is easily)fjt
300 1105 1769 13 (implemented and can be used in a variety of solicited or unsolicited polling mechanisms.)fjt
300 1004 1876 14 (It should be recognized that clock synchronization requires by its nature long periods and mult)fjt
2176 1004 75 0 (iple)fjt
300 945 1911 13 (comparisons in order to maintain accurate timekeeping. While only a few measurements are usual)fjt
2211 945 39 0 (ly)fjt
300 887 1884 17 (adequate to reliably determine local time to within a second or so, periods of many hours and doz)fjt
2184 887 66 0 (ens)fjt
300 829 1871 15 (of measurements are required to resolve oscillator drift and maintain local time to the order )fjt
2171 829 79 1 (of a)fjt
300 771 1950 14 (millisecond. Thus, the accuracy achieved is directly dependent on the time taken to achieve it.)fjt
300 712 1897 12 (Fortunately, the frequency of measurements can be quite low and almost always non-intrusive)fjt
2197 712 53 1 ( to)fjt
300 654 447 2 (normal net operations.)fjt
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300 457 1950 18 (In what may be the most common client/server model a client sends an NTP message to one or more)fjt
300 399 1740 13 (servers and processes the replies as received. The server interchanges addresses and ports, )fjt
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950 2113 650 3 (Figure 1. Implementation Model)fjt
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300 2841 1913 12 (certain fields in the message, recalculates the checksum and returns the message immediatel)fjt
2213 2841 38 0 (y.)fjt
300 2782 1810 15 (Information included in the NTP message allows the client to determine the server time with )fjt
2110 2782 141 0 (respect)fjt
300 2724 1770 14 (to local time and adjust the local clock accordingly. In addition, the message includes inf)fjt
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300 2608 306 1 (several servers.)fjt
300 2514 1820 16 (While the client/server model may suffice for use on local nets involving a public server and p)fjt
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300 2455 1848 13 (many workstation clients, the full generality of NTP requires distributed participation of a nu)fjt
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300 2397 1864 10 (of client/servers or peers arranged in a dynamically reconfigurable, hierarchically distrib)fjt
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300 2339 1748 10 (configuration. It also requires sophisticated algorithms for association management, data )fjt
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300 2281 561 3 (tion and local-clock control.)fjt
300 2180 169 1 (Figure 1)fjt
469 2180 1782 12 ( shows an implementation model for a time-server host including three processes sharing)fjt
300 2121 1826 15 (a partitioned data base, with a partition dedicated to each peer, and interconnected by a me)fjt
2126 2121 124 0 (ssage-)fjt
300 2063 1892 13 (passing system. The transmit process, driven by independent timers for each peer, collects inf)fjt
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300 2005 1950 16 (mation in the data base and sends NTP messages to the peers. Each message contains the local)fjt
300 1947 1950 12 (timestamp when the message is sent, together with previously received timestamps and other)fjt
300 1888 1950 11 (information necessary to determine the hierarchy and manage the association. The message)fjt
300 1830 1815 16 (transmission rate is determined by the accuracy required of the local clock, as well as the est)fjt
2115 1830 135 0 (imated)fjt
300 1772 450 3 (accuracies of its peers.)fjt
300 1671 1950 14 (The receive process receives NTP messages and perhaps messages in other protocols, as well as)fjt
300 1613 1950 12 (information from directly connected timecode receivers. When an NTP message is received, the)fjt
300 1554 1909 16 (offset between the peer clock and the local clock is computed and incorporated into the data ba)fjt
2209 1554 41 0 (se)fjt
300 1496 1792 13 (along with other information useful for error estimation and peer selection. A filtering al)fjt
2092 1496 158 0 (gorithm)fjt
300 1438 1461 10 (described in Section 4 improves the estimates by discarding inferior data.)fjt
300 1337 1781 16 (The update procedure is initiated upon receipt of a message and at other times. It processes t)fjt
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300 1279 1950 17 (data from each peer and selects the best one using the algorithms of Section 4. This may involve)fjt
300 1220 1851 16 (many observations of a few peers or a few observations of many peers, depending on the accur)fjt
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300 1061 1898 14 (The local-clock process operates upon the offset data produced by the update procedure and adju)fjt
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300 1003 1950 16 (the phase and frequency of the local clock using the mechanisms described in Section 5. This may)fjt
300 945 1863 17 (result in either a step-change or a gradual slew adjustment of the local clock to reduce the offs)fjt
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300 887 1878 17 (zero. The local clock provides a stable source of time information to other users of the system )fjt
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300 828 782 5 (for subsequent reference by NTP itself.)fjt
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300 632 1917 13 (The synchronization subnet is a connected network of primary and secondary time servers, clien)fjt
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300 457 1805 13 (possibly via other secondary servers, from a primary server over network paths possibly shar)fjt
2105 457 145 1 (ed with)fjt
300 399 1793 13 (other services. Under normal circumstances it is intended that the synchronization subnet of )fjt
2093 399 158 0 (primary)fjt
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300 2841 1892 10 (and secondary servers assumes a hierarchical-master-slave configuration with the primary serv)fjt
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300 2782 1861 14 (at the root and secondary servers of decreasing accuracy at successive levels toward the leave)fjt
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300 2686 1886 12 (Following conventions established by the telephone industry [34], the accuracy of each serve)fjt
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300 2628 1950 15 (defined by a number called its stratum, with the topmost level \(primary servers\) assigned as one)fjt
300 2569 1950 14 (and each level downwards \(secondary servers\) in the hierarchy assigned as one greater than the)fjt
300 2511 1851 10 (preceding level. With current technology and available timecode receivers, single-sample accur)fjt
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300 2453 1810 17 (in the order of a millisecond can be achieved at the network interface of a primary server. Acc)fjt
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300 2231 1925 12 (As the stratum increases from one, the single-sample accuracies achievable will degrade dependin)fjt
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300 2172 1950 14 (on the network paths and local-clock stabilities. In order to avoid the tedious calculations [7])fjt
300 2114 1812 14 (necessary to estimate errors in each specific configuration, it is useful to assume the measu)fjt
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300 2056 1875 14 (errors accumulate approximately in proportion to the total roundtrip path delay to the root of)fjt
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300 1998 1340 7 (synchronization subnet, which is called the synchronizing distance.)fjt
300 1892 1950 13 (Again drawing from the experience of the telephone industry, which learned such lessons at)fjt
300 1834 1950 12 (considerable cost [45], the synchronization subnet should be organized to produce the highest)fjt
300 1775 1950 14 (accuracy, but must never be allowed to form a loop, regardless of synchronizing distance. An)fjt
300 1717 1629 12 (additional factor is that each increment in stratum involves a potentially unreliable ti)fjt
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300 1659 1833 12 (introduces additional measurement errors. The selection algorithm used in NTP uses a vari)fjt
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300 1601 1870 11 (the Bellman-Ford distributed routing algorithm [37] to compute the minimum-weight spanning t)fjt
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300 1542 1925 15 (rooted on the primary servers. With the foregoing factors in mind, the distance metric was chose)fjt
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300 1484 1929 14 (using the stratum number as the high-order bits and synchronizing distance as the low-order bits.)fjt
300 1378 1903 12 (As a result of this design, the subnet reconfigures automatically in a hierarchical-master-sla)fjt
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300 1320 1950 15 (configuration to produce the most accurate and reliable time, even when one or more primary or)fjt
300 1262 1914 15 (secondary servers or the network paths between them fail. This includes the case where all norm)fjt
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300 1145 1834 13 (synchronization distances\) on a possibly partitioned subnet fail, but one or more backup pr)fjt
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300 1029 1887 12 (operation. However, should all primary servers throughout the subnet fail, the remaining second)fjt
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300 971 1950 11 (servers will synchronize among themselves while distances ratchet upwards to a preselected)fjt
300 912 1925 12 (maximum infinity due to the well-known properties of the Bellman-Ford algorithm. Upon reachin)fjt
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300 854 1878 17 (the maximum on all paths, a server will drop off the subnet and free-run using its last determi)fjt
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300 796 1950 13 (time and frequency. Since these computations are expected to be very precise, especially in)fjt
300 738 1864 13 (frequency, even extended outage periods should result in timekeeping errors not greater than a)fjt
2164 738 86 1 ( few)fjt
300 679 424 2 (milliseconds per day.)fjt
300 574 1796 13 (In the case of multiple primary servers, the spanning-tree computation will usually select th)fjt
2096 574 154 1 (e server)fjt
300 516 1889 12 (at minimum synchronization distance. However, when these servers are at approximately the sa)fjt
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300 457 1950 14 (distance, the computation may result in random selections among them as the result of normal)fjt
300 399 1845 13 (dispersive delays. Ordinarily, this does not degrade accuracy as long as any discrepancy bet)fjt
2145 399 105 0 (ween)fjt
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300 2841 1793 15 (the primary servers is small compared to the synchronization distance. If not, the filter and s)fjt
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300 2570 1906 15 (For many years the most important use of time information was for worldwide navigation and spa)fjt
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300 2511 1928 14 (science, which depend on astronomical observations of the Sun, Moon and stars [32]. Sidereal tim)fjt
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300 2453 1878 17 (is based on the transit of stars across the celestial meridian of an observer. The mean sidereal )fjt
2178 2453 72 0 (day)fjt
300 2395 1950 17 (is 23 hours, 56 minutes and 4.09 seconds, but is not uniform due to variations in Earth orbit.)fjt
300 2337 1934 17 (Ephemeris time is based on tables with which a standard time interval such as the tropical year )fjt
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300 2278 1934 15 (one complete revolution of the Earth around the Sun - can be determined through observations o)fjt
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300 2220 1950 15 (the Sun, Moon and planets. In 1958 the standard second was defined as 1/31,556,925.9747 of the)fjt
300 2162 1928 16 (tropical year that began this century. On this scale the tropical year is 365.2421987 days and th)fjt
2228 2162 22 0 (e)fjt
300 2104 1950 16 (lunar month - one complete revolution of the Moon around the Earth - is 29.53059 days; however,)fjt
300 2045 1823 18 (the actual tropical year can be determined only to an accuracy of about 50 ms and has been incr)fjt
2123 2045 127 0 (easing)fjt
300 1987 505 5 (by about 5.3 ms per year.)fjt
300 1863 1950 20 (In order to measure the span of the universe or the decay of the proton, it is necessary to have a)fjt
300 1805 1852 10 (standard day numbering plan. Accordingly, the International Astronomical Union has adopte)fjt
2152 1805 98 1 (d the)fjt
300 1746 1950 15 (use of the standard second and Julian Day Number \(JDN\) to date cosmological events and related)fjt
300 1688 1950 14 (phenomena. The standard day consists of 86,400 standard seconds, where time is expressed as a)fjt
300 1630 1950 16 (fraction of the whole day, and the standard year consists of 365.25 standard days. In the scheme)fjt
300 1572 1950 15 (devised in 1583 by the French scholar Joseph Julius Scaliger and named after his father, Julius)fjt
300 1513 885 6 (Caesar Scaliger, JDN 0.0 corresponds to 12)fjt
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300 1397 1950 15 (while the years of the Christian Era \(AD\) are reckoned according to the Gregorian calendar \(see)fjt
300 1339 1950 19 (next section\). Since there is no year zero or day zero and 1 BC is a leap year, JDN 1,721,426.0)fjt
300 1280 352 2 (corresponds to 12)fjt
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673 1280 1577 16 ( on the first day of the Christian Era, 1 January 1 AD. The Modified Julian Date)fjt
300 1222 1911 16 (\(MJD\), which is sometimes used to represent dates near our own era in conventional time and wi)fjt
2211 1222 39 0 (th)fjt
300 1164 1021 9 (fewer digits, is defined as MJD = JD - 2,400,000.5.)fjt
300 1040 1925 13 (In 1967 the standard second was redefined as 9,192,631,770 periods of the radiation correspondin)fjt
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300 982 1896 16 (to the transition between the two hyperfine levels of the ground state of the cesium-133 atom [)fjt
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300 923 1876 15 (Since 1972 the time and frequency standards of the world have been based on International Ato)fjt
2176 923 75 0 (mic)fjt
300 865 1950 14 (Time \(TAI\), which is defined in terms of the standard second and currently maintained using)fjt
300 807 1313 11 (multiple cesium-beam clocks to an accuracy of a few parts in 10)fjt
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1654 807 596 4 (. The Bureau International de)fjt
300 749 1950 12 (l'Heure \(BIH\) uses astronomical observations provided by the U.S. Naval Observatory and other)fjt
300 690 1934 11 (observatories to determine Coordinated Universal Time \(UTC\). Starting from apparent mean sola)fjt
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300 632 1851 14 (time as observed, the UT0 timescale is determined using corrections for Earth orbit and inclin)fjt
2151 632 100 0 (ation)fjt
300 574 1867 14 (\(the Equation of Time, as used by sundials\), the UT1 \(navigator's\) timescale by adding correct)fjt
2167 574 83 0 (ions)fjt
300 516 1855 13 (for polar migration and the UT2 timescale by adding corrections for known periodicity variat)fjt
2155 516 96 0 (ions.)fjt
300 457 1950 15 (While standard frequencies are based on TAI, conventional civil time is based on UT1, which is)fjt
300 399 1950 16 (presently slowing relative to TAI by a fraction of a second per year. When the magnitude of)fjt
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300 2409 1889 16 (correction approaches 0.7 second, a leap second is inserted or deleted in the UTC timescale on )fjt
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300 2350 594 5 (last day of June or December.)fjt
300 2263 1914 15 (For the most precise coordination and timestamping of events since 1972, it is necessary to kno)fjt
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300 2205 1950 15 (when leap seconds are implemented in UTC and how the seconds are numbered. As specified in)fjt
300 2147 1950 15 (CCIR Report 517, which is reproduced in [1], a leap second is inserted following second 23:59:59)fjt
300 2088 1950 18 (on the last day of June or December and becomes second 23:59:60 of that day. A leap second would)fjt
300 2030 1950 15 (be deleted by omitting second 23:59:59 on one of these days, although this has never happened.)fjt
300 1972 1750 15 (Leap seconds were inserted prior to 1 January 1989 on the occasions listed in Table 1)fjt
2050 1972 201 1 ( \(courtesy)fjt
300 1914 1878 13 (U.S. Naval Observatory\). Published BIH corrections consist not only of leap seconds, which re)fjt
2178 1914 72 0 (sult)fjt
300 1855 1950 13 (in step discontinuities relative to TAI, but 100-ms UT1 adjustments called DUT1, which provide)fjt
300 1797 1048 6 (increased accuracy for navigation and space science.)fjt
300 1710 826 8 (The NTP timescale is based on UTC. At 0)fjt
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1126 1734 21 0 (h)fjt
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1147 1710 1103 9 ( 1 January 1972 \(MJD 41,318.0\) the NTP timescale was)fjt
300 1651 1481 10 (set to 2,272,060,800, representing the number of standard seconds since 0)fjt
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1781 1675 21 0 (h)fjt
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1801 1651 449 4 ( 1 January 1900 \(MJD)fjt
300 1593 1852 15 (15,021.0\). The insertion of leap seconds in UTC does not affect the NTP oscillator itself, onl)fjt
2152 1593 99 1 (y the)fjt
300 1535 1859 11 (correspondence with conventional civil time. However, since the only institutional memory a)fjt
2159 1535 75 0 (vail)fjt
2234 1535 17 0 (-)fjt
300 1477 1950 18 (able to NTP is the UTC broadcast services, the NTP timescale is in effect reset to UTC as each)fjt
300 1418 1950 15 (offset estimate is computed. When a leap second is inserted in UTC and subsequently in NTP,)fjt
300 1360 1950 17 (knowledge of all previous leap seconds is lost. Thus, if a clock synchronized to NTP in early 1989)fjt
300 1302 1861 17 (was used to establish the time of an event that occurred in early 1972 without correction, it w)fjt
2161 1302 89 0 (ould)fjt
300 1244 492 3 (be fourteen seconds late.)fjt
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300 1155 83 0 (2.4.)fjt
(r)27 (a)29 (d)30 (n)26 (e)13 (l)27 (a)35 (C)13 ( )31 (P)30 (T)35 (N)12 ( )27 (e)30 (h)29 (T)13 ( )0 17 383 1155 fet
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300 1068 1862 14 (The calendar systems used in the ancient world reflect the agricultural, political and ritual n)fjt
2162 1068 88 0 (eeds)fjt
300 1010 1818 11 (characteristic of the societies in which they flourished. Astronomical observations to establ)fjt
2118 1010 132 1 (ish the)fjt
300 951 1950 17 (winter and summer solstices were in use three to four millennia ago. By the 14th century BC the)fjt
300 893 1950 17 (Shang Chinese had established the solar year as 365.25 days and the lunar month as 29.5 days. The)fjt
300 835 1928 17 (lunisolar calendar, in which the ritual month is based on the Moon and the agricultural year on th)fjt
2228 835 22 0 (e)fjt
300 777 1950 14 (Sun, was used throughout the ancient Near East \(except Egypt\) and Greece from the third)fjt
300 718 1886 14 (millennium BC. Early calendars used either thirteen lunar months of 28 days or twelve alternat)fjt
2186 718 64 0 (ing)fjt
300 660 1950 16 (lunar months of 29 and 30 days and haphazard means to reconcile the 354/364-day lunar year with)fjt
300 602 588 4 (the 365-day vague solar year.)fjt
300 514 1950 14 (The ancient Egyptian lunisolar calendar had twelve 30-day lunar months, but was guided by the)fjt
300 456 1804 15 (seasonal appearance of the star Sirius \(Sothis\). In order to reconcile this calendar with the sol)fjt
2104 456 146 1 (ar year,)fjt
300 398 1885 16 (a civil calendar was invented by adding five intercalary days for a total of 365 days. However)fjt
2185 398 65 1 (, in)fjt
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650 2899 1901 2899 1901 2898 650 2898 fa
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650 2899 651 2899 651 2587 650 2587 fa
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1901 2899 1900 2899 1900 2587 1901 2587 fa
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726 2838 204 1 (June 1972)fjt
1142 2838 205 1 (Dec. 1972)fjt
1558 2838 205 1 (Dec. 1973)fjt
726 2778 205 1 (Dec. 1974)fjt
1142 2778 205 1 (Dec. 1975)fjt
1558 2778 205 1 (Dec. 1976)fjt
726 2718 205 1 (Dec. 1977)fjt
1142 2718 205 1 (Dec. 1978)fjt
1558 2718 205 1 (Dec. 1979)fjt
726 2658 204 1 (June 1981)fjt
1142 2658 204 1 (June 1982)fjt
1558 2658 204 1 (June 1983)fjt
726 2598 204 1 (June 1985)fjt
1142 2598 205 1 (Dec. 1987)fjt
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871 2496 807 5 (Table 1. Dates of Leap-Second Insertion)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
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300 2841 1912 17 (time it was observed that the civil year was about one-fourth day shorter than the actual solar ye)fjt
2212 2841 39 0 (ar)fjt
300 2782 1928 17 (and thus would precess relative to it over a 1460-year cycle called the Sothic cycle. Along with th)fjt
2228 2782 22 0 (e)fjt
300 2724 1911 15 (Shang Chinese, the ancient Egyptians had thus established the solar year at 365.25 days, or with)fjt
2211 2724 39 0 (in)fjt
300 2666 1950 17 (about 11 minutes of the present measured value. In 432 BC, about a century after the Chinese had)fjt
300 2608 1950 15 (done so, the Greek astronomer Meton calculated there were 110 29-day lunar months and 125 30-day)fjt
300 2549 1950 19 (lunar months for a total of 235 lunar months in 6940 solar days, or just over 19 years. The 19-year)fjt
300 2491 1950 15 (cycle, called the Metonic cycle, established the lunar month at 29.532 solar days, or within about)fjt
300 2433 864 6 (two minutes of the present measured value.)fjt
300 2321 1950 18 (The Roman republican calendar was based on a lunar year and by 50 BC was eight weeks out of)fjt
300 2263 1928 14 (step with the solar year. Julius Caesar invited the Alexandrian astronomer Sosigenes to redesign th)fjt
2228 2263 22 0 (e)fjt
300 2205 1854 18 (calendar, which led to the adoption in 46 BC of the Julian calendar. This calendar is based on a)fjt
2154 2205 96 1 ( year)fjt
300 2147 1870 17 (of 365.25 days and has an intercalary day inserted every four years. However, for the first 36 y)fjt
2170 2147 80 0 (ears)fjt
300 2088 1860 14 (an intercalary day was mistakenly inserted every three years instead of every four. The result)fjt
2160 2088 90 1 ( was)fjt
300 2030 1817 16 (12 intercalary days instead of nine, and a series of corrections that was not complete until 8)fjt
2117 2030 97 1 ( AD.)fjt
300 1894 1934 14 (The seven-day Sumerian week was introduced only in the fourth century AD by Emperor Constan)fjt
2234 1894 17 0 (-)fjt
300 1836 1892 16 (tine I. During the Roman era a 15-year census cycle, called the Indiction cycle, was instituted )fjt
2192 1836 58 0 (for)fjt
300 1778 1869 12 (taxation purposes. The sequence of day-names for consecutive occurrences of a particular da)fjt
2169 1778 81 1 (y of)fjt
300 1719 1889 18 (the year does not recur for 28 years, called the solar cycle. Thus, the least common multiple of )fjt
2189 1719 61 0 (the)fjt
300 1661 1934 14 (28-year solar cycle, 19-year Metonic cycle and 15-year Indiction cycle results in a grand 7980-yea)fjt
2234 1661 17 0 (r)fjt
300 1603 1934 16 (supercycle called the Julian Era, which began in 4713 BC. A particular combination of the day o)fjt
2234 1603 17 0 (f)fjt
300 1545 1950 19 (the week, day of the year, phase of the Moon and round of the census will recur beginning in 3268)fjt
300 1486 85 0 (AD.)fjt
300 1351 1893 17 (By 1545 the discrepancy in the Julian year relative to the solar year had accumulated to ten da)fjt
2193 1351 57 0 (ys.)fjt
300 1292 1950 12 (In 1582, following suggestions by the astronomers Christopher Clavius and Luigi Lilio, Pope)fjt
300 1234 1903 16 (Gregory XIII issued a papal bull which decreed, among other things, that the solar year would cons)fjt
2203 1234 47 0 (ist)fjt
300 1176 1931 15 (of 365.2422 days. In order to more closely approximate the new value, only those centennial year)fjt
2231 1176 19 0 (s)fjt
300 1118 1950 15 (divisible by 400 would be leap years, while the remaining centennial years would not, making the)fjt
300 1059 1950 14 (actual value 365.2425, or within about 26 seconds of the current measured value. While the)fjt
300 1001 1950 16 (Gregorian calendar is in use throughout most of the world today, some countries did not adopt it)fjt
300 943 699 5 (until early in the twentieth century.)fjt
300 807 1889 13 (While it remains a fascinating field for time historians, the above narrative provides conclus)fjt
2189 807 61 0 (ive)fjt
300 749 1925 12 (evidence that conjugating calendar dates of significant events and assigning NTP timestamps t)fjt
2225 749 25 0 (o)fjt
300 690 1876 15 (them is approximate at best. In principle, reliable dating of such events requires only an accu)fjt
2176 690 74 0 (rate)fjt
300 632 1903 16 (count of the days relative to some globally alarming event, such as a comet passage or superno)fjt
2203 632 47 0 (va)fjt
300 574 1734 11 (explosion; however, only historically persistent and politically stable societies, such as )fjt
2034 574 217 1 (the ancient)fjt
300 516 1950 15 (Chinese and Egyptian, and especially the classic Maya, possessed the means and will to do so.)fjt
300 457 1862 14 (Therefore, intercalary dating is considered beyond the scope of this specification and NTP is b)fjt
2162 457 88 0 (ased)fjt
300 399 888 5 (solely on the Julian-Day numbering scheme.)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 2841 83 0 (2.5.)fjt
(n)30 (o)13 (i)15 (t)27 (a)29 (n)13 (i)43 (m)27 (e)27 (s)27 (s)12 (i)35 (D)13 ( )27 (y)27 (c)29 (n)27 (e)29 (u)30 (q)27 (e)18 (r)29 (F)13 ( )30 (d)29 (n)27 (a)13 ( )26 (e)44 (m)12 (i)29 (T)13 ( )0 33 383 2841 fet
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300 2751 1853 14 (In order that atomic and civil time can be coordinated throughout the world, national administra)fjt
2153 2751 97 0 (tions)fjt
300 2692 1950 12 (operate primary time and frequency standards and maintain TAI and UTC cooperatively by)fjt
300 2634 1887 13 (observing various radio broadcasts and through occasional use of portable atomic clocks. A prim)fjt
2187 2634 64 0 (ary)fjt
300 2576 1829 11 (frequency standard is an oscillator that can maintain extremely precise frequency relativ)fjt
2129 2576 121 2 (e to a)fjt
300 2518 1854 14 (physical phenomenon, such as a transition in the orbital states of an electron. Presently avai)fjt
2154 2518 97 0 (lable)fjt
300 2459 1912 15 (atomic oscillators are based on the transitions of the hydrogen, cesium and rubidium atoms and a)fjt
2212 2459 39 0 (re)fjt
300 2401 1160 9 (capable of maintaining reliable agreement to the order of 10)fjt
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1460 2425 55 0 (-13)fjt
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1516 2401 735 5 ( when operated in multiple ensembles)fjt
300 2343 1173 7 (at various national standards laboratories \(see Section 5.1\).)fjt
300 2250 1896 15 (Most seafaring nations of the world operate some sort of broadcast time service for the purpose)fjt
2196 2250 54 1 ( of)fjt
300 2192 1934 12 (calibrating chronographs, which are used in conjunction with ephemeris data to determine naviga)fjt
2234 2192 17 0 (-)fjt
300 2134 1855 13 (tional position. In many countries the service is primitive and limited to seconds-pips broadca)fjt
2155 2134 95 1 (st by)fjt
300 2075 1950 13 (marine communication stations at certain hours. For instance, a chronograph error of one second)fjt
300 2017 1623 12 (represents a longitudinal position error of about 0.23 nautical mile at the Equator.)fjt
300 1924 1950 13 (The U.S. National Institute of Standards and Technology \(NIST - formerly National Bureau of)fjt
300 1866 1815 13 (Standards\) operates three radio services for the distribution of primary time and frequency st)fjt
2115 1866 136 0 (andard)fjt
300 1808 1895 13 (information. One of these uses high-frequency \(HF or CCIR band 7\) transmissions on frequenc)fjt
2195 1808 55 0 (ies)fjt
300 1750 1950 17 (of 2.5, 5, 10, 15 and 20 MHz from Fort Collins, CO \(WWV\), and Kauai, HI \(WWVH\). Signal)fjt
300 1691 1950 14 (propagation is usually by reflection from the upper ionospheric layers, which vary in height and)fjt
300 1633 1950 13 (composition throughout the day and season and result in unpredictable delay variations at the)fjt
300 1575 1826 18 (receiver. The timecode is transmitted over a 60-second interval at a data rate of 1 bps using a 1)fjt
2126 1575 125 0 (00-Hz)fjt
300 1517 1887 14 (subcarrier on the broadcast signal. While these transmissions and those of Canada \(CHU\) and ot)fjt
2187 1517 64 0 (her)fjt
300 1458 1848 13 (countries can be received over large areas in the western hemisphere, reliable frequency compar)fjt
2148 1458 102 0 (isons)fjt
300 1400 710 8 (can be made only to the order of 10)fjt
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1010 1424 35 0 (-7)fjt
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1045 1400 1206 11 ( and time accuracies are limited to the order of a millisecond)fjt
300 1342 71 0 ([1].)fjt
300 1249 1950 16 (A second service operated by NIST is the low-frequency \(LF or CCIR band 5\) transmissions on 60)fjt
300 1191 1950 14 (kHz from Boulder, CO \(WWVB\), which can be received over the continental U.S. and adjacent)fjt
300 1132 1785 13 (coastal areas. Signal propagation is via the lower ionospheric layers, which are relatively st)fjt
2085 1132 165 1 (able and)fjt
300 1074 1851 13 (have predictable diurnal variations in height. The timecode is transmitted over a 60-second int)fjt
2151 1074 99 0 (erval)fjt
300 1016 1950 15 (at a rate of 1 pps using periodic reductions in carrier power. With appropriate receiving and)fjt
300 958 1925 10 (averaging techniques and corrections for diurnal and seasonal propagation effects, frequenc)fjt
2225 958 25 0 (y)fjt
300 899 499 3 (comparisons to within 10)fjt
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799 923 55 0 (-11)fjt
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854 899 1396 13 ( are possible and time accuracies of from a few to 50 microseconds can)fjt
300 841 1794 15 (be obtained [1]. However, there is only one station and it operates at modest power levels.)fjt
300 749 1950 14 (The third service operated by NIST uses ultra-high frequency \(UHF or CCIR band 9\) transmissions)fjt
300 690 1950 12 (on 468 MHz from the Geosynchronous Orbiting Environmental Satellite \(GOES\). The timecode is)fjt
300 632 1815 13 (interleaved with messages used to interrogate remote sensors and consists of 60 4-bit binary)fjt
2115 632 136 0 (-coded)fjt
300 574 1903 13 (decimal words transmitted over an interval of 30 seconds. The timecode information includes t)fjt
2203 574 47 0 (he)fjt
300 516 1839 15 (UTC time of year, satellite position and UTC correction. There is some speculation on the cont)fjt
2139 516 111 0 (inued)fjt
300 457 1950 13 (operation of GOES, especially if the LORAN-C [16] and Global Positioning System \(GPS\) [17])fjt
300 399 1889 13 (radiopositioning systems operated by other U.S. agencies continue to evolve as expected. While )fjt
2189 399 61 0 (the)fjt
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1024 2979 504 2 (Network Time Protocol)fjt
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300 2841 1950 14 (OMEGA [3] radionavigation system operated by the U.S. Navy and other countries can in principle)fjt
300 2782 1889 13 (provide worldwide frequency and time distribution, this system is unlikely to long survive )fjt
2189 2782 61 0 (the)fjt
300 2724 642 3 (operational deployment of GPS.)fjt
300 2635 1906 15 (Note that the current formats used by NIST radio broadcast services [5] do not include provisio)fjt
2206 2635 44 0 (ns)fjt
300 2577 1846 14 (for advance notice of leap seconds, so this information must be determined from other sources.)fjt
2146 2577 104 1 ( NTP)fjt
300 2518 1931 12 (includes provisions to distribute advance warnings of leap seconds using the leap-indicator bit)fjt
2231 2518 19 0 (s)fjt
300 2460 1848 18 (described in Section 3. The protocol is designed so that these bits can be set manually at the pri)fjt
2148 2460 102 0 (mary)fjt
300 2402 1873 12 (time servers and then automatically distributed throughout the synchronization subnet to all o)fjt
2173 2402 77 0 (ther)fjt
300 2344 761 6 (time servers as described in Section 5.)fjt
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300 2251 42 0 (3.)fjt
(l)30 (o)27 (c)29 (o)16 (t)29 (o)18 (r)33 (P)12 ( )27 (e)43 (m)13 (i)29 (T)13 ( )26 (k)19 (r)29 (o)38 (w)15 (t)27 (e)35 (N)13 ( )0 22 350 2251 fet
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300 2162 1777 14 (This section consists of a formal definition of the Network Time Protocol, including its data)fjt
2077 2162 173 1 ( formats,)fjt
300 2104 1826 11 (entities, state variables, events and event-processing procedures. The specification is based )fjt
2126 2104 124 1 (on the)fjt
300 2045 1826 16 (implementation model illustrated in Figure 1, but it is not intended that this model is the on)fjt
2126 2045 124 1 (ly one)fjt
300 1987 1793 14 (upon which a specification can be based. In particular, the specification is intended to illust)fjt
2093 1987 157 1 (rate and)fjt
300 1929 1918 17 (clarify the intrinsic operations of NTP, as well as to serve as a foundation for a more rigorou)fjt
2218 1929 32 0 (s,)fjt
300 1871 867 3 (comprehensive and verifiable specification.)fjt
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300 1778 83 0 (3.1.)fjt
(s)15 (t)27 (a)43 (m)18 (r)30 (o)29 (F)13 ( )27 (a)15 (t)27 (a)35 (D)13 ( )0 13 383 1778 fet
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300 1689 1950 10 (All mathematical operations expressed or implied herein are in two's-complement, fixed-point)fjt
300 1630 1925 13 (arithmetic. Data are specified as integer or fixed-point quantities, with bits numbered from zer)fjt
2225 1630 25 0 (o)fjt
300 1572 1876 12 (starting at the left, or high-order, position. Since various implementations may scale extern)fjt
2176 1572 75 0 (ally)fjt
300 1514 1831 12 (derived quantities for internal use, neither the precision nor decimal-point placement for fixed)fjt
2131 1514 119 0 (-point)fjt
300 1456 1852 12 (quantities is specified. Unless specified otherwise, all quantities are unsigned and may occup)fjt
2152 1456 99 1 (y the)fjt
300 1397 1950 14 (full field width with an implied zero preceding bit zero. Hardware and software packages designed)fjt
300 1339 1870 14 (to work with signed quantities will thus yield surprising results when the most significant \(sign)fjt
2170 1339 80 1 (\) bit)fjt
300 1281 1773 13 (is set. It is suggested that externally derived, unsigned fixed-point quantities such as times)fjt
2073 1281 177 1 (tamps be)fjt
300 1223 1763 16 (shifted right one bit for internal use, since the precision represented by the full field width )fjt
2063 1223 187 1 (is seldom)fjt
300 1164 175 0 (justified.)fjt
300 1073 1950 15 (Since NTP timestamps are cherished data and, in fact, represent the main product of the protocol,)fjt
300 1015 1950 13 (a special timestamp format has been established. NTP timestamps are represented as a 64-bit)fjt
300 956 1052 7 (unsigned fixed-point number, in seconds relative to 0)fjt
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1373 956 878 10 ( on 1 January 1900. The integer part is in the)fjt
300 898 1729 16 (first 32 bits and the fraction part in the last 32 bits. This format allows convenient multipl)fjt
2029 898 221 0 (e-precision)fjt
300 840 1950 11 (arithmetic and conversion to Time Protocol representation \(seconds\), but does complicate the)fjt
300 782 1950 10 (conversion to ICMP Timestamp message representation \(milliseconds\). The precision of this)fjt
300 723 1950 13 (representation is about 200 picoseconds, which should be adequate for even the most exotic)fjt
300 665 272 0 (requirements.)fjt
300 574 1950 15 (Timestamps are determined by copying the current value of the local clock to a timestamp when)fjt
300 516 1895 16 (some significant event, such as the arrival of a message, occurs. In order to maintain the high)fjt
2195 516 55 0 (est)fjt
300 457 1805 16 (accuracy, it is important that this be done as close to the hardware or software driver associat)fjt
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300 399 1837 13 (the event as possible. In particular, departure timestamps should be redetermined for each link)fjt
2137 399 113 0 (-level)fjt
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300 2841 1950 15 (retransmission. In some cases a particular timestamp may not be available, such as when the host)fjt
300 2782 1800 18 (is rebooted or the protocol first starts up. In these cases the 64-bit field is set to zero, indica)fjt
2100 2782 150 1 (ting the)fjt
300 2724 582 4 (value is invalid or undefined.)fjt
300 2634 1854 19 (Note that since some time in 1968 the most significant bit \(bit 0 of the integer part\) has been se)fjt
2154 2634 96 1 (t and)fjt
300 2575 1936 18 (that the 64-bit field will overflow some time in 2036. Should NTP be in use in 2036, some externa)fjt
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300 2517 1950 16 (means will be necessary to qualify time relative to 1900 and time relative to 2036 \(and other)fjt
300 2459 1914 13 (multiples of 136 years\). Timestamped data requiring such qualification will be so precious th)fjt
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300 2401 1950 11 (appropriate means should be readily available. There will exist an 200-picosecond interval,)fjt
300 2342 1899 15 (henceforth ignored, every 136 years when the 64-bit field will be zero and thus considered inval)fjt
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300 2156 1950 15 (Following is a summary of the various state variables and parameters used by the protocol. They)fjt
300 2098 1808 13 (are separated into classes of system variables, which relate to the operating system environm)fjt
2108 2098 143 1 (ent and)fjt
300 2040 1950 12 (local-clock mechanism; peer variables, which represent the state of the protocol machine specific)fjt
300 1982 1918 14 (to each peer; packet variables, which represent the contents of the NTP message; and parameter)fjt
2218 1982 32 0 (s,)fjt
300 1923 1803 11 (which represent fixed configuration constants for all implementations of the current versi)fjt
2103 1923 147 1 (on. For)fjt
300 1865 1864 17 (each class the description of the variable is followed by its name and the procedure or value w)fjt
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300 1807 1914 15 (controls it. Note that variables are in lower case, while parameters are in upper case. Addition)fjt
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300 1559 1950 15 (The following variables are common to two or more of the system, peer and packet classes.)fjt
300 1500 1789 12 (Additional variables are specific to the optional authentication mechanism as described in A)fjt
2089 1500 161 0 (ppendix)fjt
300 1442 46 0 (C.)fjt
300 1346 1950 11 (Peer Address \(peer.srcadr, pkt.srcadr\), Peer Port \(peer.srcport, pkt.srcport\): These are the 32-bit)fjt
375 1286 1175 9 (Internet address and 16-bit port number of the remote host.)fjt
300 1189 1923 11 (Local Address \(peer.dstadr, pkt.dstadr\), Local Port \(peer.dstport, pkt.dstport\): These are the 32-b)fjt
2223 1189 28 0 (it)fjt
375 1129 1875 15 (Internet address and 16-bit port number of the local host. They are included among the state)fjt
375 1069 692 3 (variables to support multi-homing.)fjt
300 973 1881 14 (Leap Indicator \(sys.leap, peer.leap, pkt.leap\): This is a two-bit code warning of an impending l)fjt
2181 973 69 0 (eap)fjt
375 913 1850 18 (second to be inserted in the NTP timescale. The bits are set before 23:59 on the day of insertio)fjt
2225 913 25 0 (n)fjt
375 853 1823 15 (and reset after 00:00 on the following day. This causes the number of seconds \(rollover interv)fjt
2198 853 52 0 (al\))fjt
375 793 1875 18 (in the day of insertion to be increased or decreased by one. In the case of primary servers the)fjt
375 733 1825 17 (bits are set by operator intervention, while in the case of secondary servers the bits are set )fjt
2200 733 50 0 (by)fjt
375 673 1527 14 (the protocol. The two bits, bit 0 and bit 1, respectively, are coded as follows:)fjt
533 579 50 0 (00)fjt
700 579 226 1 (no warning)fjt
533 519 50 0 (01)fjt
700 519 531 4 (last minute has 61 seconds)fjt
533 459 50 0 (10)fjt
700 459 547 4 (last minute has 59 seconds\))fjt
533 399 50 0 (11)fjt
700 399 821 4 (alarm condition \(clock not synchronized\))fjt
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375 2839 705 6 (In all except the alarm condition \(11)fjt
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1080 2834 21 0 (2)fjt
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1101 2839 1150 10 (\), NTP itself does nothing with these bits, except pass them)fjt
375 2781 1875 15 (on to the time-conversion routines that are not part of NTP. The alarm condition occurs when,)fjt
375 2722 1875 16 (for whatever reason, the local clock is not synchronized, such as when first coming up or after)fjt
375 2664 1307 9 (an extended period when no outside reference source is available.)fjt
300 2561 1950 12 (Mode \(peer.hmode, pkt.pmode\): This is an integer indicating the association mode, with values)fjt
375 2501 348 2 (coded as follows:)fjt
533 2404 25 0 (0)fjt
700 2404 229 0 (unspecified)fjt
533 2344 25 0 (1)fjt
700 2344 341 1 (symmetric active)fjt
533 2284 25 0 (2)fjt
700 2284 369 1 (symmetric passive)fjt
533 2224 25 0 (3)fjt
700 2224 110 0 (client)fjt
533 2164 25 0 (4)fjt
700 2164 121 0 (server)fjt
533 2104 25 0 (5)fjt
700 2104 191 0 (broadcast)fjt
533 2044 25 0 (6)fjt
700 2044 655 4 (reserved for future NTP versions)fjt
533 1984 25 0 (7)fjt
700 1984 468 3 (reserved for private use)fjt
300 1888 1823 12 (Stratum \(sys.stratum, peer.stratum, pkt.stratum\): This is an integer indicating the stratum of th)fjt
2123 1888 127 1 (e local)fjt
375 1828 752 5 (clock, with values defined as follows:)fjt
533 1731 25 0 (0)fjt
700 1731 229 0 (unspecified)fjt
533 1671 25 0 (1)fjt
700 1671 720 4 (primary reference \(e.g., radio clock\))fjt
533 1611 117 0 (2-255)fjt
700 1611 612 3 (secondary reference \(via NTP\))fjt
375 1515 1875 15 (For comparison purposes a value of zero is considered greater than any other value. Note that)fjt
375 1457 1875 15 (the maximum value of the integer encoded as a packet variable is limited by the parameter)fjt
375 1398 638 4 (NTP.INFIN, currently set to 15.)fjt
300 1295 1812 13 (Peer Poll Interval \(peer.ppoll, pkt.ppoll\): This is a signed integer indicating the minimum i)fjt
2112 1295 138 0 (nterval)fjt
375 1235 1875 18 (between messages sent by the peer, in seconds as a power of two. For instance, a value of six)fjt
375 1175 881 6 (indicates a minimum interval of 64 seconds.)fjt
300 1071 1950 10 (Precision \(sys.precision, peer.precision, pkt.precision\): This is a signed integer indicating the)fjt
375 1011 1875 17 (precision of the local clock, in seconds to the nearest power of two. For instance, a 50-Hz or)fjt
375 951 1875 12 (60-Hz line-frequency clock would be assigned the value -6, while a 1000-Hz crystal-controlled)fjt
375 891 773 6 (clock would be assigned the value -10.)fjt
300 787 1912 9 (Synchronizing Distance \(sys.distance, peer.distance, pkt.distance\): This is a fixed-point numb)fjt
2212 787 39 0 (er)fjt
375 727 1454 10 (indicating the estimated roundtrip delay to the primary clock, in seconds.)fjt
300 623 1831 8 (Synchronizing Dispersion \(sys.dispersion, peer.dispersion, pkt.dispersion\): This is a fixed)fjt
2131 623 119 0 (-point)fjt
375 563 1518 10 (number indicating the estimated dispersion to the primary clock, in seconds.)fjt
300 459 1847 11 (Reference Clock Identifier \(sys.refid, peer.refid, pkt.refid\): This is a 32-bit code identifyin)fjt
2147 459 103 1 (g the)fjt
375 399 1777 14 (particular reference clock. In the case of stratum 0 \(unspecified\) or stratum 1 \(primary refere)fjt
2152 399 98 0 (nce\),)fjt
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375 2033 1875 13 (this is a four-octet, left-justified, zero-padded ASCII string, for example \(see Appendix A for)fjt
375 1973 405 1 (comprehensive list\):)fjt
651 1853 1900 1853 1900 1852 651 1852 fa
gs eofill gr
650 1923 1901 1923 1901 1922 650 1922 fa
gs eofill gr
650 1541 1901 1541 1901 1542 650 1542 fa
gs eofill gr
650 1923 651 1923 651 1541 650 1541 fa
gs eofill gr
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gs eofill gr
700 1862 158 0 (Stratum)fjt
978 1862 105 0 (Code)fjt
1255 1862 177 0 (Meaning)fjt
700 1792 25 0 (0)fjt
978 1792 105 0 (DCN)fjt
1255 1792 440 2 (DCN routing protocol)fjt
700 1732 25 0 (0)fjt
978 1732 86 0 (TSP)fjt
1255 1732 365 2 (TSP time protocol)fjt
700 1672 25 0 (1)fjt
978 1672 163 0 (WWVB)fjt
1255 1672 529 4 (WWVB LF \(band 5\) radio)fjt
700 1612 25 0 (1)fjt
978 1612 130 0 (GOES)fjt
1255 1612 589 4 (GOES UHF \(band 9\) satellite)fjt
700 1552 25 0 (1)fjt
978 1552 130 0 (WWV)fjt
1255 1552 489 3 (WWV HF \(band 7\)radio)fjt
375 1450 1802 15 (In the case of type 2 and greater \(secondary reference\) this is the four-octet Internet addres)fjt
2177 1450 73 1 (s of)fjt
375 1391 366 2 (the reference host.)fjt
300 1296 1851 11 (Reference Timestamp \(sys.reftime, peer.reftime, pkt.reftime\): This is the local time, in times)fjt
2151 1296 100 0 (tamp)fjt
375 1236 1875 15 (format, when the local clock was last updated. If the local clock has never been synchronized,)fjt
375 1176 337 3 (the value is zero.)fjt
300 1079 1950 14 (Originate Timestamp \(peer.org, pkt.org\): This is the local time, in timestamp format, at the peer)fjt
375 1019 1875 17 (when its latest NTP message was sent. If the peer becomes unreachable the value is set to zero.)fjt
300 922 1917 14 (Receive Timestamp \(peer.rec, pkt.rec\): This is the local time, in timestamp format, when the late)fjt
2217 922 33 0 (st)fjt
375 862 1846 16 (NTP message from the peer arrived. If the peer becomes unreachable the value is set to zero.)fjt
300 766 1928 14 (Transmit Timestamp \(peer.xmt, pkt.xmt\): This is the local time, in timestamp format, at which th)fjt
2228 766 22 0 (e)fjt
375 706 687 4 (NTP message departed the sender.)fjt
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300 516 1864 15 (Table 2 shows the complete set of system variables. In addition to the common variables descr)fjt
2164 516 86 0 (ibed)fjt
300 457 1903 14 (previously, the following variables are used by the operating system in order to synchronize t)fjt
2203 457 47 0 (he)fjt
300 399 229 1 (local clock.)fjt
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576 2838 349 1 (System Variables)fjt
1218 2838 119 0 (Name)fjt
1646 2838 202 0 (Procedure)fjt
576 2768 292 1 (Leap Indicator)fjt
1218 2768 159 0 (sys.leap)fjt
1646 2768 253 1 (clock update)fjt
576 2708 158 0 (Stratum)fjt
1218 2708 225 0 (sys.stratum)fjt
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576 2648 185 0 (Precision)fjt
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576 2588 477 1 (Synchronizing Distance)fjt
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1646 2528 253 1 (clock update)fjt
576 2468 530 2 (Reference Clock Identifier)fjt
1218 2468 170 0 (sys.refid)fjt
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576 2408 438 1 (Reference Timestamp)fjt
1218 2408 219 0 (sys.reftime)fjt
1646 2408 253 1 (clock update)fjt
576 2348 284 1 (Logical Clock)fjt
1218 2348 184 0 (sys.clock)fjt
1646 2348 253 1 (clock update)fjt
576 2288 231 1 (Clock Hold)fjt
1218 2288 165 0 (sys.hold)fjt
1646 2288 253 1 (clock update)fjt
576 2218 270 1 (Clock Source)fjt
1218 2218 162 0 (sys.peer)fjt
1646 2218 177 0 (selection)fjt
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1013 2120 525 3 (Table 2. System Variables)fjt
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300 818 1903 15 (Local Clock \(sys.clock\): This is the current local time, in timestamp format. Local time is deriv)fjt
2203 818 47 0 (ed)fjt
375 758 1875 14 (from the hardware clock of the particular machine and increments at intervals depending on the)fjt
375 698 1875 10 (design used. An appropriate design, including slewing and drift-compensation mechanisms, is)fjt
375 638 454 3 (described in Section 5.)fjt
300 519 1934 16 (Clock Hold \(sys.hold\): This is a counter used to avoid premature resetting of the local clock afte)fjt
2234 519 17 0 (r)fjt
375 459 1863 18 (the clock is reset to a new value, rather than being slewed gradually. Once set to a nonzero value)fjt
2238 459 13 0 (,)fjt
375 399 1567 10 (this counter decrements at one-second intervals until reaching zero, then stops.)fjt
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576 2838 291 1 (Peer Variables)fjt
1218 2838 119 0 (Name)fjt
1646 2838 202 0 (Procedure)fjt
576 2768 300 1 (Configured Bit)fjt
1218 2768 225 0 (peer.config)fjt
1646 2768 251 0 (initialization)fjt
576 2708 472 1 (Authentication Enabled)fjt
576 2648 61 0 (Bit)fjt
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576 2588 369 1 (Authentication Bit)fjt
1218 2588 280 0 (peer.authentic)fjt
1646 2588 143 0 (receive)fjt
576 2528 264 1 (Peer Address)fjt
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576 2468 184 1 (Peer Port)fjt
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576 2408 289 1 (Local Address)fjt
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576 2348 209 1 (Local Port)fjt
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576 2288 292 1 (Leap Indicator)fjt
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576 2228 223 1 (Host Mode)fjt
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576 2168 158 0 (Stratum)fjt
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576 2108 348 2 (Peer Poll Interval)fjt
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576 2048 354 2 (Host Poll Interval)fjt
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1646 2048 223 1 (poll update)fjt
576 1988 185 0 (Precision)fjt
1218 1988 280 0 (peer.precision)fjt
1646 1988 130 0 (packet)fjt
576 1928 477 1 (Synchronizing Distance)fjt
1218 1928 261 0 (peer.distance)fjt
1646 1928 130 0 (packet)fjt
576 1868 519 1 (Synchronizing Dispersion)fjt
1218 1868 303 0 (peer.dispersion)fjt
1646 1868 130 0 (packet)fjt
576 1808 530 2 (Reference Clock Identifier)fjt
1218 1808 192 0 (peer.refid)fjt
1646 1808 130 0 (packet)fjt
576 1748 438 1 (Reference Timestamp)fjt
1218 1748 241 0 (peer.reftime)fjt
1646 1748 130 0 (packet)fjt
576 1688 424 1 (Originate Timestamp)fjt
1218 1688 165 0 (peer.org)fjt
1646 1688 251 1 (packet, clear)fjt
576 1628 396 1 (Receive Timestamp)fjt
1218 1628 159 0 (peer.rec)fjt
1646 1628 251 1 (packet, clear)fjt
576 1568 416 1 (Transmit Timestamp)fjt
1218 1568 176 0 (peer.xmt)fjt
1646 1568 284 1 (transmit, clear)fjt
576 1508 429 1 (Reachability Register)fjt
1218 1508 206 0 (peer.reach)fjt
1646 1508 251 1 (packet, trans)fjt
1897 1508 17 0 (-)fjt
1646 1448 188 1 (mit, clear)fjt
576 1388 390 2 (Valid Data Counter)fjt
1218 1388 198 0 (peer.valid)fjt
1646 1388 251 1 (packet, trans)fjt
1897 1388 17 0 (-)fjt
1646 1328 188 1 (mit, clear)fjt
576 1268 222 1 (Peer Timer)fjt
1218 1268 203 0 (peer.timer)fjt
1646 1268 265 1 (receive, trans)fjt
1911 1268 17 0 (-)fjt
1646 1208 314 2 (mit, poll update)fjt
576 1148 286 1 (Filter Register)fjt
1218 1148 194 0 (peer.filter)fjt
1646 1148 218 1 (filter, clear)fjt
576 1088 305 1 (Delay Estimate)fjt
1218 1088 261 0 (peer.estdelay)fjt
1646 1088 96 0 (filter)fjt
576 1018 310 1 (Offset Estimate)fjt
1218 1018 266 0 (peer.estoffset)fjt
1646 1018 96 0 (filter)fjt
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1042 913 467 3 (Table 3. Peer Variables)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
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300 1678 1815 14 (Clock Source \(sys.peer\): This is a selector identifying the current clock source. Usually this )fjt
2115 1678 136 1 (will be)fjt
375 1618 1046 8 (a pointer to a structure containing the peer variables.)fjt
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300 1522 125 0 (3.2.3.)fjt
(s)27 (e)12 (l)30 (b)27 (a)12 (i)19 (r)26 (a)30 (V)13 ( )18 (r)27 (e)26 (e)33 (P)12 ( )0 15 425 1522 fet
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300 1425 1925 15 (Table 3 shows the complete set of peer variables. In addition to the common variables describe)fjt
2225 1425 25 0 (d)fjt
300 1367 1912 12 (previously, the following variables are used by the peer management and measurement function)fjt
2212 1367 32 0 (s.)fjt
300 1271 1950 13 (Configured Bit \(peer.config\): This is a bit indicating that the association was created from)fjt
375 1211 1806 11 (configuration information and should not be demobilized if the peer becomes unreachable.)fjt
300 1113 1892 12 (Authentication Enabled Bit \(peer.authenable\): This is a bit indicating that the association is)fjt
2192 1113 58 1 ( to)fjt
375 1053 1875 15 (operate in the authenticated mode. It may be set to one only if the optional authentication)fjt
375 993 1066 6 (mechanism described in Appendix C is implemented.)fjt
300 895 1812 13 (Authenticated Bit \(peer.authentic\): This is a bit indicating that the last message received fr)fjt
2112 895 138 1 (om the)fjt
375 835 1811 15 (peer has been correctly authenticated. It may be set to one only if the optional authenticat)fjt
2186 835 64 0 (ion)fjt
375 775 1066 6 (mechanism described in Appendix C is implemented.)fjt
300 677 1881 14 (Host Poll Interval \(peer.hpoll\): This is a signed integer used to indicate the interval betw)fjt
2181 677 69 0 (een)fjt
375 617 1875 17 (messages transmitted to the peer, in seconds as a power of two. For instance, a value of six)fjt
375 557 881 6 (indicates a minimum interval of 64 seconds.)fjt
300 459 1950 13 (Reachability Register \(peer.reach\): This is a shift register of NTP.WINDOW bits used to determine)fjt
375 399 1861 14 (the reachability status of the peer, with bits entering from the least significant \(rightmost\) end)fjt
2236 399 13 0 (.)fjt
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576 2838 335 1 (Packet Variables)fjt
1218 2838 119 0 (Name)fjt
1646 2838 202 0 (Procedure)fjt
576 2768 264 1 (Peer Address)fjt
1218 2768 198 0 (pkt.srcadr)fjt
1646 2768 163 0 (transmit)fjt
576 2708 184 1 (Peer Port)fjt
1218 2708 214 0 (pkt.srcport)fjt
1646 2708 163 0 (transmit)fjt
576 2648 289 1 (Local Address)fjt
1218 2648 198 0 (pkt.dstadr)fjt
1646 2648 163 0 (transmit)fjt
576 2588 209 1 (Local Port)fjt
1218 2588 215 0 (pkt.dstport)fjt
1646 2588 163 0 (transmit)fjt
576 2528 292 1 (Leap Indicator)fjt
1218 2528 159 0 (pkt.leap)fjt
1646 2528 163 0 (transmit)fjt
576 2468 333 1 (Version Number)fjt
1218 2468 223 0 (pkt.version)fjt
1646 2468 163 0 (transmit)fjt
576 2408 217 1 (Peer Mode)fjt
1218 2408 212 0 (pkt.pmode)fjt
1646 2408 163 0 (transmit)fjt
576 2348 158 0 (Stratum)fjt
1218 2348 225 0 (pkt.stratum)fjt
1646 2348 163 0 (transmit)fjt
576 2288 348 2 (Peer Poll Interval)fjt
1218 2288 179 0 (pkt.ppoll)fjt
1646 2288 163 0 (transmit)fjt
576 2228 185 0 (Precision)fjt
1218 2228 259 0 (pkt.precision)fjt
1646 2228 163 0 (transmit)fjt
576 2168 477 1 (Synchronizing Distance)fjt
1218 2168 239 0 (pkt.distance)fjt
1646 2168 163 0 (transmit)fjt
576 2108 519 1 (Synchronizing Dispersion)fjt
1218 2108 281 0 (pkt.dispersion)fjt
1646 2108 163 0 (transmit)fjt
576 2048 530 2 (Reference Clock Identifier)fjt
1218 2048 170 0 (pkt.refid)fjt
1646 2048 163 0 (transmit)fjt
576 1988 438 1 (Reference Timestamp)fjt
1218 1988 220 0 (pkt.reftime)fjt
1646 1988 163 0 (transmit)fjt
576 1928 424 1 (Originate Timestamp)fjt
1218 1928 143 0 (pkt.org)fjt
1646 1928 163 0 (transmit)fjt
576 1868 396 1 (Receive Timestamp)fjt
1218 1868 137 0 (pkt.rec)fjt
1646 1868 163 0 (transmit)fjt
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1020 1766 511 3 (Table 4. Packet Variables)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
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300 1734 1881 14 (Valid Data Counter \(peer.valid\): This is an integer counter used to determine the interval betw)fjt
2181 1734 69 0 (een)fjt
375 1674 372 2 (valid data updates.)fjt
300 1584 1862 14 (Peer Timer \(peer.timer\): This is an integer counter used to control the interval between transm)fjt
2162 1584 88 0 (itted)fjt
375 1524 307 1 (NTP messages.)fjt
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300 1436 125 0 (3.2.4.)fjt
(s)27 (e)13 (l)29 (b)27 (a)13 (i)18 (r)27 (a)29 (V)13 ( )15 (t)27 (e)27 (k)27 (c)26 (a)33 (P)12 ( )0 17 425 1436 fet
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300 1348 1864 15 (Table 4 shows the complete set of packet variables. In addition to the common variables descr)fjt
2164 1348 86 0 (ibed)fjt
300 1289 941 5 (previously, the following variables are defined.)fjt
300 1200 1832 13 (Version Number \(pkt.version\): This is an integer indicating the version number of the sender)fjt
2132 1200 118 1 (. NTP)fjt
375 1140 1875 13 (messages will always be sent with the current version number NTP.VERSION and will always)fjt
375 1080 1875 13 (be accepted if the version number matches NTP.VERSION. Exceptions may be advised on a)fjt
375 1020 1875 12 (case-by-case basis at times when the version number is changed. Specific guidelines for)fjt
375 960 1850 12 (interoperation between this version and previous versions of NTP are summarized in Appendi)fjt
2225 960 25 0 (x)fjt
375 900 49 0 (D.)fjt
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300 812 125 0 (3.2.5.)fjt
(s)27 (e)12 (l)30 (b)27 (a)12 (i)19 (r)26 (a)30 (V)13 ( )18 (r)27 (e)15 (t)13 (l)13 (i)29 (F)13 ( )27 (k)26 (c)30 (o)13 (l)35 (C)12 ( )0 23 425 812 fet
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300 724 1741 14 (When the filter and selection algorithms suggested in Section 4 are used, the following stat)fjt
2041 724 209 1 (e variables)fjt
(v)10 ( )22 (e)19 (s)22 (e)25 (h)14 (t)9 ( )17 (f)25 (o)9 ( )14 (t)22 (e)19 (s)10 ( )22 (e)25 (n)25 (o)9 ( )19 (s)14 (i)10 ( )22 (e)16 (r)22 (e)25 (h)31 (T)9 ( )13 (.)25 (y)13 (l)20 (s)25 (u)25 (o)13 (i)25 (v)22 (e)17 (r)25 (p)9 ( )25 (d)22 (e)25 (b)14 (i)17 (r)22 (c)19 (s)22 (e)25 (d)9 ( )20 (s)22 (e)13 (l)25 (b)22 (a)14 (i)17 (r)22 (a)25 (v)9 ( )17 (r)22 (e)22 (e)25 (p)9 ( )22 (e)25 (h)14 (t)9 ( )25 (o)14 (t)10 ( )25 (n)25 (o)13 (i)14 (t)14 (i)25 (d)25 (d)22 (a)9 ( )25 (n)14 (i)10 ( )9 ( )25 (d)22 (e)25 (n)14 (i)17 (f)22 (e)25 (d)9 ( )22 (e)17 (r)22 (a)0 96 300 666 fet
(s)22 (e)14 (l)25 (b)22 (a)14 (i)16 (r)22 (a)0 8 2096 666 fet
300 607 1100 9 (for every peer operating in an active mode \(see below\).)fjt
300 518 1892 14 (Filter Register \(peer.filter\): This is a shift register of PEER.SHIFT stages, where each stage sto)fjt
2192 518 58 0 (res)fjt
375 458 1875 14 (a tuple consisting of the measured delay together with the measured offset associated with a)fjt
375 398 1798 10 (single observation. Delay/offset observations enter from the least significant \(rightmost\) r)fjt
2173 398 78 0 (ight)fjt
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576 2838 221 0 (Parameters)fjt
1138 2838 119 0 (Name)fjt
1699 2838 119 0 (Value)fjt
576 2768 333 1 (Version Number)fjt
1138 2768 323 0 (NTP.VERSION)fjt
1699 2768 25 0 (2)fjt
576 2708 190 1 (NTP Port)fjt
1138 2708 234 0 (NTP.PORT)fjt
1699 2708 75 0 (123)fjt
576 2648 261 1 (Max Stratum)fjt
1138 2648 240 0 (NTP.INFIN)fjt
1699 2648 50 0 (15)fjt
576 2588 318 2 (Max Clock Age)fjt
1138 2588 326 0 (NTP.MAXAGE)fjt
1699 2588 213 1 (86,400 sec)fjt
576 2528 215 1 (Max Skew)fjt
1138 2528 334 0 (NTP.MAXSKW)fjt
1699 2528 138 1 (.01 sec)fjt
576 2468 269 1 (Min Distance)fjt
1138 2468 314 0 (NTP.MINDIST)fjt
1699 2468 138 1 (.02 sec)fjt
576 2408 407 2 (Min Polling Interval)fjt
1138 2408 328 0 (NTP.MINPOLL)fjt
1699 2408 196 2 (6 \(64 sec\))fjt
576 2348 415 2 (Max Polling Interval)fjt
1138 2348 348 0 (NTP.MAXPOLL)fjt
1699 2348 271 2 (10 \(1024 sec\))fjt
576 2288 429 1 (Reachability Register)fjt
576 2228 86 0 (Size)fjt
1138 2288 325 0 (NTP.WINDOW)fjt
1699 2288 25 0 (8)fjt
576 2168 384 2 (Max Select Weight)fjt
1138 2168 337 0 (NTP.MAXWGT)fjt
1699 2168 25 0 (8)fjt
576 2108 323 2 (Max Select Size)fjt
1138 2108 328 0 (NTP.MAXLIST)fjt
1699 2108 25 0 (5)fjt
576 2048 353 2 (Max Select Strata)fjt
1138 2048 351 0 (NTP.MAXSTRA)fjt
1699 2048 25 0 (2)fjt
576 1988 280 1 (Select Weight)fjt
1138 1988 290 0 (NTP.SELECT)fjt
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1699 2012 21 0 (3)fjt
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1720 1988 14 0 (/)fjt
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1734 1983 21 0 (4)fjt
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576 1928 206 1 (Filter Size)fjt
1138 1928 273 0 (PEER.SHIFT)fjt
1699 1928 117 2 (4 or 8)fjt
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1077 1828 396 2 (Table 5. Parameters)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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375 2839 1875 13 (and are shifted towards the most significant \(leftmost\) end and eventually discarded as new)fjt
375 2779 1859 14 (observations arrive. The register is cleared to zeros when \(a\) the peer becomes unreachable o)fjt
2234 2779 17 0 (r)fjt
375 2719 1823 17 (\(b\) the local clock has just been reset so as to cause a significant discontinuity in local time.)fjt
300 2625 1914 12 (Delay Estimate \(peer.estdelay\): This is a fixed-point number indicating the latest delay estima)fjt
2214 2625 36 0 (te)fjt
375 2565 665 5 (output from the filter, in seconds.)fjt
300 2467 1837 12 (Offset Estimate \(peer.estoffset\): This is a signed, fixed-point number indicating the latest )fjt
2137 2467 113 0 (offset)fjt
375 2407 843 6 (estimate output from the filter, in seconds.)fjt
300 2309 1867 11 (Dispersion Estimate \(peer.estdisp\): This is a fixed-point number indicating the latest disper)fjt
2167 2309 83 0 (sion)fjt
375 2249 843 6 (estimate output from the filter, in seconds.)fjt
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300 2153 125 0 (3.2.6.)fjt
(s)18 (r)27 (e)16 (t)26 (e)44 (m)26 (a)19 (r)26 (a)33 (P)12 ( )0 11 425 2153 fet
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300 2056 151 1 (Table 5)fjt
451 2056 1799 13 ( shows the parameters assumed for all implementations operating in the Internet system. It)fjt
300 1998 1950 15 (is necessary to agree on the values for these parameters in order to avoid unnecessary network)fjt
300 1940 778 4 (overheads and stable peer associations.)fjt
300 1843 1740 11 (Version Number \(NTP.VERSION\): This is the NTP version number, currently two \(2\).)fjt
300 1745 1950 14 (NTP Port \(NTP.PORT\): This is the port number \(123\) assigned by the Internet Assigned Numbers)fjt
375 1685 364 2 (Authority to NTP.)fjt
300 1587 1950 15 (Maximum Strata \(NTP.INFIN\): This is the maximum stratum value that can be encoded as a packet)fjt
375 1527 1732 12 (variable, also interpreted as "infinity" or unreachable by the routing algorithm and curre)fjt
2107 1527 144 1 (ntly set)fjt
375 1467 1875 19 (to 15. In some cases it may be desirable to set NTP.INFIN to a lower value in order to avoid)fjt
375 1407 726 3 (long, unstable synchronizing chains.)fjt
300 1309 1950 12 (Maximum Clock Age \(NTP.MAXAGE\): This is the maximum interval, in seconds, a reference)fjt
375 1249 1774 16 (clock will be considered valid after its last update, currently set to 86,400 seconds \(one full )fjt
2149 1249 101 0 (day\).)fjt
300 1151 1950 13 (Maximum Skew \(NTP.MAXSKW\): This is the maximum allowance for the skew between the local)fjt
375 1091 1875 13 (clock and a peer clock over the maximum update interval determined by NTP.MAXPOLL \(1024)fjt
375 1031 755 5 (seconds\), currently set to .01 seconds.)fjt
300 933 1950 10 (Minimum Distance \(NTP.MINDIST\): This is the minimum synchronization distance between the)fjt
375 873 881 8 (host and a peer, currently set to .02 seconds.)fjt
300 775 1911 12 (Minimum Polling Interval \(NTP.MINPOLL\): This is the minimum polling interval, in seconds )fjt
2211 775 39 0 (to)fjt
375 715 1850 17 (the power of two, allowed by any peer of the Internet system, currently set to 6 \(64 seconds\).)fjt
300 617 1911 12 (Maximum Polling Interval \(NTP.MAXPOLL\): This is the maximum polling interval, in seconds )fjt
2211 617 39 0 (to)fjt
375 557 1827 17 (the power of two, allowed by any peer of the Internet system, currently set to 10 \(1024 second)fjt
2202 557 48 0 (s\).)fjt
(r)23 (e)14 (t)19 (s)14 (i)25 (g)23 (e)33 (R)25 ( )25 (y)14 (t)14 (i)14 (l)14 (i)26 (b)22 (a)25 (h)22 (c)23 (a)22 (e)33 (R)25 ( )23 (e)25 (h)14 (t)25 ( )17 (f)25 (o)25 ( )22 (e)22 (z)14 (i)20 (s)25 ( )22 (e)25 (h)14 (t)25 ( )20 (s)14 (i)25 ( )19 (s)14 (i)26 (h)30 (T)25 ( )14 (:)17 (\))47 (W)37 (O)36 (D)36 (N)17 (I)47 (W)13 (.)28 (P)31 (T)36 (N)17 (\()25 ( )22 (e)22 (z)14 (i)28 (S)25 ( )17 (r)22 (e)14 (t)20 (s)14 (i)25 (g)23 (e)34 (R)25 ( )25 (y)14 (t)15 (i)14 (l)14 (i)26 (b)22 (a)26 (h)22 (c)23 (a)22 (e)34 (R)0 86 300 459 fet
375 399 612 4 (\(peer.reach\), currently set to 8.)fjt
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300 2839 1950 11 (Maximum Select Weight \(NTP.MAXWGT\): When the selection algorithm suggested in Section 4)fjt
375 2779 1363 11 (is used, this is the maximum allowable dispersion, currently set to 8.)fjt
300 2687 1950 12 (Maximum Select Size \(NTP.MAXLIST\): When the selection algorithm suggested in Section 4 is)fjt
375 2627 1373 13 (used, this is the maximum size of the selection list, currently set to 5.)fjt
300 2535 1950 11 (Maximum Select Strata \(NTP.MAXSTRA\): When the selection algorithm suggested in Section 4)fjt
375 2475 1787 15 (is used, this is the maximum number of strata represented in the selection list, currently s)fjt
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375 2415 38 0 (2.)fjt
300 2323 1950 13 (Select Weight \(NTP.SELECT\): When the selection algorithm suggested in Section 4 is used, this)fjt
375 2263 1219 11 (is the weight used to compute the dispersion, currently set to )fjt
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300 2171 1840 15 (Filter Size \(PEER.SHIFT\): When the filter algorithm suggested in Section 4 is used, this is th)fjt
2140 2171 110 1 (e size)fjt
375 2111 1743 12 (of the Clock Filter \(peer.filter\) shift register. For crystal-stabilized oscillators a value )fjt
2118 2111 132 2 (of 8 is)fjt
375 2051 1759 12 (suggested, while for mains-frequency oscillators a value of 4 is suggested. Additional cons)fjt
2134 2051 99 0 (idera)fjt
2234 2051 17 0 (-)fjt
375 1991 556 5 (tions are given in Section 5.)fjt
300 1899 1925 11 (Maximum Filter Dispersion \(PEER.MAXDISP\): When the filter algorithm suggested in Section )fjt
2225 1899 25 0 (4)fjt
375 1839 1352 11 (is used, this is the maximum dispersion, currently set to 64 seconds.)fjt
300 1747 1950 12 (Filter Threshold \(PEER.THRESHOLD\): When the filter algorithm suggested in Section 4 is used,)fjt
375 1687 1801 15 (this is the threshold used to determine whether to increase or decrease the polling interval. W)fjt
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375 1627 206 3 (a value of )fjt
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616 1622 21 0 (2)fjt
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636 1627 1614 14 ( is suggested, the value may be changed to suit local conditions on particular peer)fjt
375 1567 118 0 (paths.)fjt
300 1475 1859 14 (Filter Weight \(PEER.FILTER\): When the filter algorithm suggested in Section 4 is used, this i)fjt
2159 1475 91 1 (s the)fjt
375 1415 1025 10 (weight used to discard noisy data. While a value of )fjt
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1455 1415 795 7 ( is suggested, the value may be changed)fjt
375 1355 945 7 (to suit local conditions on particular peer paths.)fjt
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300 1265 83 0 (3.3.)fjt
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300 1177 1950 16 (An NTP association is formed when two peers exchange messages and one or both of them create)fjt
300 1119 1841 12 (and maintain an instantiation of the protocol machine, called an association. The associatio)fjt
2141 1119 109 1 (n can)fjt
300 1060 1877 14 (operate in one of five modes as indicated by the host-mode variable \(peer.hmode\): symmetric act)fjt
2177 1060 73 0 (ive,)fjt
300 1002 1548 10 (symmetric passive, client, server and broadcast, which are defined as follows:)fjt
300 911 1950 14 (Symmetric Active \(1\): A host operating in this mode sends periodic messages regardless of the)fjt
375 851 1875 15 (reachability state or stratum of its peer. By operating in this mode the host announces its)fjt
375 791 1199 8 (willingness to synchronize and be synchronized by the peer.)fjt
300 699 1775 14 (Symmetric Passive \(2\): This type of association is ordinarily created upon arrival of a mess)fjt
2075 699 176 1 (age from)fjt
375 639 1853 17 (a peer operating in the symmetric active mode and persists only as long as the peer is reachabl)fjt
2228 639 22 0 (e)fjt
375 579 1875 16 (and operating at a stratum level less than or equal to the host; otherwise, the association is)fjt
375 519 1814 14 (dissolved. However, the association will always persist until at least one message has been s)fjt
2189 519 61 0 (ent)fjt
375 459 1875 15 (in reply. By operating in this mode the host announces its willingness to synchronize and be)fjt
375 399 512 3 (synchronized by the peer.)fjt
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300 2839 1825 14 (Client \(3\): A host operating in this mode sends periodic messages regardless of the reachabilit)fjt
2125 2839 125 1 (y state)fjt
375 2779 1875 16 (or stratum of its peer. By operating in this mode the host, usually a LAN workstation, announces)fjt
375 2719 1390 11 (its willingness to be synchronized by, but not to synchronize the peer.)fjt
300 2627 1821 15 (Server \(4\): This type of association is ordinarily created upon arrival of a client request m)fjt
2121 2627 130 0 (essage)fjt
375 2567 1875 16 (and exists only in order to reply to that request, after which the association is dissolved. By)fjt
375 2507 1875 14 (operating in this mode the host, usually a LAN time server, announces its willingness to)fjt
375 2447 1041 8 (synchronize, but not to be synchronized by the peer.)fjt
300 2356 1870 14 (Broadcast \(5\): A host operating in this mode sends periodic messages regardless of the reachab)fjt
2170 2356 80 0 (ility)fjt
375 2296 1875 17 (state or stratum of the peers. By operating in this mode the host, usually a LAN time server)fjt
375 2236 1875 12 (operating on a high-speed broadcast medium, announces its willingness to synchronize all of)fjt
375 2176 1070 10 (the peers, but not to be synchronized by any of them..)fjt
300 2086 1950 14 (The peer mode can be determined explicitly from the packet-mode variable \(pkt.pmode\) if it is)fjt
300 2027 1818 12 (nonzero and implicitly from the source port \(pkt.srcport\) and destination port \(pkt.dstport\) va)fjt
2118 2027 132 0 (riables)fjt
300 1969 1950 16 (if it is zero. For the case where pkt.pmode is zero, included for compatibility with previous NTP)fjt
300 1911 984 7 (versions, the peer mode is determined as follows:)fjt
526 1790 2024 1790 2024 1789 526 1789 fa
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576 1549 311 1 (not NTP.PORT)fjt
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1574 1549 239 1 (not possible)fjt
300 1451 1914 15 (Note that it is not possible in this case to distinguish between symmetric active and symmetr)fjt
2214 1451 36 0 (ic)fjt
300 1393 1950 14 (passive modes. Use of the pkt.pmode and NTP.PORT variables in this way is not recommended)fjt
300 1334 1207 11 (and may not be supported in future versions of the protocol. )fjt
300 1245 1934 16 (A host operating in client mode occasionally sends an NTP message to a host operating in serve)fjt
2234 1245 17 0 (r)fjt
300 1187 1864 12 (mode, perhaps right after rebooting and at periodic intervals thereafter. The server respond)fjt
2164 1187 87 1 (s by)fjt
300 1129 1820 11 (simply interchanging addresses and ports, filling in the required information and returni)fjt
2120 1129 130 1 (ng the)fjt
300 1071 1789 13 (message to the client. Servers need retain no state information between client requests, whil)fjt
2089 1071 161 1 (e clients)fjt
300 1012 1887 15 (are free to manage the intervals between sending NTP messages to suit local conditions. In th)fjt
2187 1012 63 0 (ese)fjt
300 954 1950 14 (modes the protocol machine described in this document can be considerably simplified to a simple)fjt
300 896 1925 9 (remote-procedure-call mechanism without significant loss of accuracy or robustness, especiall)fjt
2225 896 25 0 (y)fjt
300 838 787 4 (when operating over high-speed LANs.)fjt
300 749 1828 10 (In the symmetric modes the client/server distinction \(almost\) disappears. Symmetric passive)fjt
2128 749 123 1 ( mode)fjt
300 690 1851 16 (is intended for use by time servers operating near the root nodes \(lowest stratum\) of the synchro)fjt
2151 690 83 0 (niza)fjt
2234 690 17 0 (-)fjt
300 632 1853 16 (tion subnet and with a relatively large number of peers on an intermittent basis. In this mod)fjt
2153 632 97 1 (e the)fjt
300 574 1862 16 (identity of the peer need not be known in advance, since the association with its state variabl)fjt
2162 574 88 1 (es is)fjt
300 516 1950 15 (created only when an NTP message arrives. Furthermore, the state storage can be reused when the)fjt
300 457 1950 15 (peer becomes unreachable or is operating at a higher stratum level and thus ineligible as a)fjt
300 399 473 1 (synchronization source.)fjt
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300 2841 1950 15 (Symmetric active mode is intended for use by time servers operating near the end nodes \(highest)fjt
300 2782 1889 13 (stratum\) of the synchronization subnet. Reliable time service can usually be maintained with t)fjt
2189 2782 61 0 (wo)fjt
300 2724 1900 19 (peers at the next lower stratum level and one peer at the same stratum level, so the rate of ongoi)fjt
2200 2724 50 0 (ng)fjt
300 2666 1801 15 (polls is usually not significant, even when connectivity is lost and error messages are being r)fjt
2101 2666 149 0 (eturned)fjt
300 2608 284 2 (for every poll.)fjt
300 2518 1950 14 (Normally, one peer operates in an active mode \(symmetric active, client or broadcast modes\) as)fjt
300 2460 1829 16 (configured by a startup file, while the other operates in a passive mode \(symmetric passive or )fjt
2129 2460 121 0 (server)fjt
300 2401 1903 14 (modes\), often without prior configuration. However, both peers can be configured to operate in t)fjt
2203 2401 47 0 (he)fjt
300 2343 1950 15 (symmetric active mode. An error condition results when both peers operate in the same mode, but)fjt
300 2285 1950 16 (not symmetric active mode. In such cases each peer will ignore messages from the other, so that)fjt
300 2227 1447 10 (prior associations, if any, will be demobilized due to reachability failure.)fjt
300 2137 1950 12 (Broadcast mode is intended for operation on high-speed LANs with numerous workstations and)fjt
300 2078 1900 16 (where the highest accuracies are not required. In the typical scenario one or more time servers )fjt
2200 2078 50 0 (on)fjt
300 2020 1931 15 (the LAN send periodic broadcasts to the workstations, which then determine the time on the basi)fjt
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300 1962 1950 16 (of a preconfigured latency in the order of a few milliseconds. As in the client/server modes the)fjt
300 1904 1950 14 (protocol machine can be considerably simplified in this mode; however, a modified form of the)fjt
300 1845 1950 14 (clock selection algorithm may prove useful in cases where multiple time servers are used for)fjt
300 1787 406 1 (enhanced reliability.)fjt
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300 1696 83 0 (3.4.)fjt
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300 1608 1850 15 (The significant events of interest in NTP occur upon expiration of a peer timer \(peer.timer\), o)fjt
2150 1608 100 1 (ne of)fjt
300 1549 1950 16 (which is dedicated to each peer with an active association, and upon arrival of an NTP message)fjt
300 1491 1950 17 (from the various peers. An event can also occur as the result of an operator command or detected)fjt
300 1433 1903 14 (system fault, such as a primary clock failure. This section describes the procedures invoked wh)fjt
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300 1375 377 2 (these events occur.)fjt
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300 1284 125 0 (3.4.1.)fjt
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300 1193 1950 14 (The transmit procedure is called when the peer timer \(peer.timer\) decrements to zero, which can)fjt
300 1135 1950 16 (occur in all modes except server mode. First, an NTP message is constructed and sent as follows)fjt
300 1077 1931 16 (\(see Appendix A for format\). The IP and UDP packet variables are copied from the peer variable)fjt
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300 1018 1361 9 (\(note the interchange of source and destination addresses and ports\):)fjt
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300 664 1950 14 (Next, the NTP packet variables are copied \(rescaled as necessary\) from the system and peer)fjt
300 606 193 0 (variables:)fjt
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(l)14 (l)25 (o)25 (p)25 (h)12 (.)17 (r)22 (e)22 (e)25 (p)12 ( )0 11 1289 2782 fet
( )25 (n)25 (o)14 (i)19 (s)14 (i)22 (c)22 (e)16 (r)25 (p)13 (.)13 (t)25 (k)25 (p)0 14 980 2724 fet
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(\254)0 1 1251 2724 fet
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(n)25 (o)14 (i)19 (s)14 (i)22 (c)22 (e)17 (r)25 (p)12 (.)19 (s)25 (y)20 (s)12 ( )0 14 1300 2724 fet
( )22 (e)22 (c)25 (n)22 (a)14 (t)19 (s)14 (i)25 (d)12 (.)14 (t)25 (k)25 (p)0 13 999 2666 fet
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(\254)0 1 1251 2666 fet
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(e)22 (c)25 (n)22 (a)14 (t)19 (s)14 (i)25 (d)12 (.)19 (s)25 (y)20 (s)12 ( )0 13 1300 2666 fet
( )25 (n)25 (o)14 (i)19 (s)16 (r)22 (e)25 (p)20 (s)13 (i)25 (d)13 (.)14 (t)25 (k)25 (p)0 15 957 2608 fet
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(\254)0 1 1251 2608 fet
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(n)25 (o)14 (i)19 (s)17 (r)22 (e)25 (p)19 (s)14 (i)25 (d)12 (.)19 (s)25 (y)20 (s)12 ( )0 15 1300 2608 fet
( )25 (d)14 (i)16 (f)22 (e)17 (r)12 (.)14 (t)25 (k)25 (p)0 10 1068 2549 fet
/tface 12 def
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(\254)0 1 1251 2549 fet
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(d)14 (i)16 (f)22 (e)17 (r)12 (.)19 (s)25 (y)20 (s)12 ( )0 10 1300 2549 fet
( )22 (e)39 (m)13 (i)14 (t)17 (f)22 (e)16 (r)13 (.)13 (t)25 (k)25 (p)0 12 1019 2491 fet
/tface 12 def
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(\254)0 1 1251 2491 fet
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(e)39 (m)14 (i)14 (t)16 (f)22 (e)17 (r)12 (.)19 (s)25 (y)20 (s)12 ( )0 12 1300 2491 fet
( )25 (g)16 (r)25 (o)13 (.)14 (t)25 (k)25 (p)0 8 1084 2433 fet
/tface 12 def
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(\254)0 1 1240 2433 fet
/tface 8 def
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(g)17 (r)25 (o)12 (.)17 (r)22 (e)22 (e)25 (p)12 ( )0 9 1289 2433 fet
( )22 (c)22 (e)16 (r)13 (.)14 (t)25 (k)25 (p)0 8 1090 2375 fet
/tface 12 def
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(\254)0 1 1240 2375 fet
/tface 8 def
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(c)22 (e)17 (r)12 (.)17 (r)22 (e)22 (e)25 (p)12 ( )0 9 1289 2375 fet
( )14 (t)39 (m)25 (x)12 (.)14 (t)25 (k)25 (p)0 8 1069 2316 fet
/tface 12 def
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(\254)0 1 1235 2316 fet
/tface 8 def
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(k)22 (c)25 (o)14 (l)22 (c)12 (.)20 (s)25 (y)19 (s)12 ( )0 10 1285 2316 fet
( )14 (t)39 (m)25 (x)12 (.)17 (r)22 (e)22 (e)25 (p)0 9 1073 2258 fet
/tface 12 def
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(\254)0 1 1261 2258 fet
/tface 8 def
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(t)39 (m)25 (x)12 (.)14 (t)25 (k)25 (p)12 ( )0 8 1311 2258 fet
300 2161 1925 13 (If message authentication is implemented the encrypt procedure \(See Appendix C\) is called t)fjt
2225 2161 25 0 (o)fjt
300 2103 1307 8 (generate the authenticator, which follows the NTP message itself.)fjt
300 1995 1836 16 (Note that the transmit timestamp \(peer.xmt\), which is updated at this time, will be used later in)fjt
2136 1995 115 1 ( order)fjt
300 1937 1863 14 (to validate the reply; thus, implementations must take care to save the value actually transmi)fjt
2163 1937 87 0 (tted.)fjt
300 1878 1950 14 (However, if message authentication is implemented it is likely that the time to compute the)fjt
300 1820 1862 10 (authentication information, which involves a crypto-checksum, can seriously affect the ov)fjt
2162 1820 88 0 (erall)fjt
300 1762 1950 10 (accuracy. Therefore, implementations should include a system state variable \(not mentioned)fjt
300 1704 1950 14 (elsewhere in this document\) which contains an offset calculated to match the expected time to)fjt
300 1645 1950 15 (compute this value and which is added to the transmit timestamp as obtained from the operating)fjt
300 1587 1950 16 (system. In addition, the order of copying the timestamps should be designed so that the time to)fjt
300 1529 1950 11 (perform the copy operations themselves does not degrade the measurement accuracy, which)fjt
300 1471 1123 9 (suggests that the variables be copied in the order shown.)fjt
300 1363 1829 14 (Next, the reachability register \(peer.reach\) is shifted one position to the left, with zero replaci)fjt
2129 1363 122 1 (ng the)fjt
300 1305 1837 14 (vacated bit. If the reachability register \(peer.reach\) is zero and the association was not confi)fjt
2137 1305 114 0 (gured)fjt
300 1246 1765 12 (by the initialization procedure \(peer.config bit set to zero\), the association is demobilize)fjt
2065 1246 185 2 (d and the)fjt
300 1188 1770 14 (transmit procedure exits. If peer.reach is zero and peer.config is set, the clear procedure is)fjt
2070 1188 180 2 ( called to)fjt
300 1130 1273 10 (purge the clock filter and reselect the clock source, if necessary.)fjt
300 1022 1729 16 (If the valid data counter \(peer.valid\) is less than two, it is incremented; otherwise, at least t)fjt
2029 1022 221 1 (wo timeout)fjt
300 964 1871 15 (intervals have passed since valid data were shifted into the filter register. If the latter case)fjt
2171 964 79 1 ( the)fjt
300 906 1818 14 (clock-filter procedure is called with zeros as the offset and delay arguments. Then, the clock)fjt
2118 906 116 0 (-selec)fjt
2234 906 17 0 (-)fjt
300 847 1764 17 (tion procedure is called to reselect the clock source, if necessary. If this results in a new clo)fjt
2064 847 186 1 (ck source)fjt
300 789 1839 12 (\(sys.peer\), the poll-update procedure is called for sys.peer with argument peer.hpoll, since th)fjt
2139 789 112 1 (e poll)fjt
300 731 1950 14 (interval for the new clock source must be clamped at NTP.MINPOLL. Note that the zeros)fjt
300 673 1779 11 (\(undefined\) argument will cause the computed dispersion to increase significantly and subs)fjt
2079 673 172 0 (equently)fjt
300 614 847 6 (affect the poll interval and clock selection.)fjt
300 507 987 7 (Next, the peer.timer is reinitialized with the value)fjt
( )16 (r)22 (e)39 (m)14 (i)14 (t)12 (.)17 (r)22 (e)22 (e)25 (p)0 11 397 399 fet
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(\254)0 1 613 399 fet
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( )25 (1)12 ( )0 3 662 399 fet
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(<)23 (<)0 2 712 399 fet
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(])30 (L)31 (L)36 (O)28 (P)36 (N)16 (I)44 (M)13 (.)28 (P)30 (T)36 (N)13 ( )12 (,)17 (\))30 (L)31 (L)36 (O)27 (P)36 (X)36 (A)45 (M)12 (.)28 (P)30 (T)36 (N)13 ( )12 (,)14 (l)14 (l)25 (o)25 (p)25 (h)12 (.)17 (r)22 (e)22 (e)25 (p)12 ( )13 (,)14 (l)13 (l)25 (o)25 (p)25 (p)13 (.)16 (r)22 (e)22 (e)25 (p)17 (\()25 (n)14 (i)38 (m)17 ([)25 (x)22 (a)39 (m)12 ( )0 59 759 399 fet
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
2065 279 186 1 (Page 22)fjt
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300 2841 1837 12 (The host-poll variable \(peer.hpoll\) is then updated as follows. If the estimated-dispersion va)fjt
2137 2841 113 0 (riable)fjt
300 2782 1950 10 (\(peer.estdisp\) is greater than the filter-threshold parameter \(PEER.THRESHOLD, currently set to)fjt
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300 2748 21 0 (1)fjt
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321 2724 14 0 (/)fjt
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335 2719 21 0 (2)fjt
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355 2724 1895 14 (\), the poll-update procedure is called with argument peer.hpoll-1 to reduce its value by one;)fjt
300 2666 1814 13 (otherwise, that procedure is called with argument peer.hpoll+1 to increase its value by one.)fjt
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300 2569 125 0 (3.4.2.)fjt
(e)18 (r)29 (u)30 (d)27 (e)26 (c)30 (o)18 (r)32 (P)13 ( )27 (e)27 (v)12 (i)27 (e)27 (c)27 (e)35 (R)12 ( )0 18 425 2569 fet
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300 2472 1928 16 (The receive procedure is executed upon arrival of an NTP message. If the version number of th)fjt
2228 2472 22 0 (e)fjt
300 2414 1950 11 (message \(pkt.version\) does not match the current version number \(NTP.VERSION\), the message)fjt
300 2356 1898 14 (is discarded and the procedure exits; however, exceptions may be advised on a case-by-case ba)fjt
2198 2356 52 0 (sis)fjt
300 2297 1890 14 (at times when the version number is changed. Next, the source and destination Internet addres)fjt
2190 2297 61 0 (ses)fjt
300 2239 1900 19 (and ports in the IP and UDP headers are matched to the correct peer. If there is a match, processi)fjt
2200 2239 50 0 (ng)fjt
300 2181 1859 17 (continues at the next step below. If there is no match a new instantiation of the protocol machi)fjt
2159 2181 92 1 (ne is)fjt
300 2123 975 6 (created and the association mobilized as follows:)fjt
( )17 (r)25 (d)22 (a)22 (c)16 (r)19 (s)13 (.)16 (r)22 (e)22 (e)25 (p)0 12 1030 2027 fet
/tface 12 def
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(\254)0 1 1261 2027 fet
/tface 8 def
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(r)25 (d)22 (a)22 (c)16 (r)20 (s)12 (.)14 (t)25 (k)25 (p)12 ( )0 11 1311 2027 fet
( )14 (t)16 (r)25 (o)25 (p)22 (c)17 (r)19 (s)13 (.)16 (r)22 (e)22 (e)25 (p)0 13 1013 1969 fet
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(\254)0 1 1261 1969 fet
/tface 8 def
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(t)17 (r)25 (o)25 (p)22 (c)16 (r)20 (s)12 (.)14 (t)25 (k)25 (p)12 ( )0 12 1311 1969 fet
( )17 (r)25 (d)22 (a)13 (t)20 (s)25 (d)12 (.)17 (r)22 (e)22 (e)25 (p)0 12 1029 1910 fet
/tface 12 def
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(\254)0 1 1261 1910 fet
/tface 8 def
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(r)25 (d)22 (a)13 (t)20 (s)25 (d)12 (.)14 (t)25 (k)25 (p)12 ( )0 11 1311 1910 fet
( )14 (t)16 (r)25 (o)25 (p)14 (t)19 (s)25 (d)13 (.)16 (r)22 (e)22 (e)25 (p)0 13 1013 1852 fet
/tface 12 def
/mpf false def
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(\254)0 1 1261 1852 fet
/tface 8 def
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(t)17 (r)25 (o)25 (p)13 (t)20 (s)25 (d)12 (.)14 (t)25 (k)25 (p)12 ( )0 12 1311 1852 fet
( )25 (g)14 (i)16 (f)25 (n)25 (o)22 (c)13 (.)16 (r)22 (e)22 (e)25 (p)0 12 1113 1794 fet
/tface 12 def
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(\254)0 1 1351 1794 fet
/tface 8 def
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(0)12 ( )0 2 1400 1794 fet
( )22 (e)14 (l)25 (b)22 (a)25 (n)22 (e)25 (h)13 (t)25 (u)22 (a)13 (.)16 (r)22 (e)22 (e)25 (p)0 16 1069 1736 fet
/tface 12 def
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(\254)0 1 1395 1736 fet
/tface 8 def
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(0)12 ( )0 2 1444 1736 fet
( )22 (c)14 (i)14 (t)25 (n)22 (e)25 (h)13 (t)25 (u)22 (a)13 (.)16 (r)22 (e)22 (e)25 (p)0 15 983 1677 fet
/tface 12 def
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(\254)0 1 1275 1677 fet
/tface 8 def
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(\))36 (w)25 (o)14 (l)22 (e)25 (b)12 ( )22 (e)22 (e)19 (s)17 (\()12 ( )0 12 1325 1677 fet
( )22 (e)25 (d)25 (o)39 (m)25 (h)13 (.)16 (r)22 (e)22 (e)25 (p)0 11 1006 1619 fet
/tface 12 def
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(\254)0 1 1252 1619 fet
/tface 8 def
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(\))36 (w)25 (o)14 (l)22 (e)25 (b)12 ( )22 (e)22 (e)20 (s)16 (\()13 ( )0 12 1301 1619 fet
( )25 (h)22 (c)22 (a)22 (e)16 (r)13 (.)16 (r)22 (e)22 (e)25 (p)0 11 1123 1561 fet
/tface 12 def
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(\254)0 1 1341 1561 fet
/tface 8 def
/mpf true def
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(0)12 ( )0 2 1390 1561 fet
( )25 (y)22 (a)13 (l)22 (e)25 (d)14 (t)19 (s)22 (e)13 (.)16 (r)22 (e)22 (e)25 (p)0 14 973 1503 fet
/tface 12 def
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(\254)0 1 1246 1503 fet
/tface 8 def
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(\))25 (d)22 (e)25 (n)14 (i)16 (f)22 (e)25 (d)25 (n)25 (u)17 (\()12 ( )25 (0)13 ( )0 14 1295 1503 fet
( )14 (t)22 (e)19 (s)16 (f)17 (f)25 (o)14 (t)19 (s)22 (e)12 (.)17 (r)22 (e)22 (e)25 (p)0 15 970 1444 fet
/tface 12 def
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(\254)0 1 1249 1444 fet
/tface 8 def
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(\))25 (d)22 (e)25 (n)14 (i)17 (f)22 (e)25 (d)25 (n)25 (u)16 (\()13 ( )25 (0)12 ( )0 14 1298 1444 fet
300 1348 1950 12 (If the optional authentication mechanism described in Appendix C is not implemented, the)fjt
300 1290 1812 13 (peer.authentic bit is ordinarily set to one, which allows non-preconfigured peers to become th)fjt
2112 1290 139 1 (e clock)fjt
300 1232 1834 16 (source. If this bit is set to zero, a non-preconfigured peer cannot become the clock source, rega)fjt
2134 1232 116 0 (rdless)fjt
300 1174 1931 13 (of stratum or mode. If the mechanism is implemented, additional variables are initialized a)fjt
2231 1174 19 0 (s)fjt
300 1115 1928 14 (described in Appendix C. The values of these variables are obtained using procedures beyond th)fjt
2228 1115 22 0 (e)fjt
300 1057 1771 17 (scope of NTP itself. Ordinarily in this case the peer.authentic bit is set to zero, so that only)fjt
2071 1057 180 1 ( properly)fjt
300 999 979 6 (authenticated peers can become the clock source.)fjt
300 903 1917 18 (If the message is from a peer operating in client mode \(3\), as determined in Section 3.3, the ho)fjt
2217 903 33 0 (st)fjt
300 845 1950 16 (mode \(peer.hmode\) is set to server mode \(4\); otherwise, it is set to symmetric passive mode \(2\).)fjt
300 786 1863 14 (This may be modified in case the access-controls suggested in Section 3.5 are implemented. Fin)fjt
2163 786 87 0 (ally,)fjt
300 728 1713 13 (the clear procedure is called to initialize the remaining peer variables. Finally, the timer)fjt
2013 728 237 1 ( mechanism)fjt
300 670 1229 8 (is armed and begins decrementing the peer timer \(peer.timer\).)fjt
300 574 1876 13 (If the authentication mechanism is implemented, the decrypt procedure \(see Appendix C\) is ca)fjt
2176 574 75 0 (lled)fjt
300 516 1801 15 (to verify the authenticator and set the peer.authentic bit. Next, if pkt.pmode is nonzero, this b)fjt
2101 516 149 0 (ecomes)fjt
300 457 1906 19 (the value of the peer mode used in the following step. If pkt.pmode is zero, the peer is a previo)fjt
2206 457 44 0 (us)fjt
300 399 1950 15 (NTP version and the peer mode is determined from the port numbers as described previously. Table)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
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300 1842 25 0 (6)fjt
325 1842 1925 15 ( shows for each combination of peer mode and host mode \(peer.hmode\) the resulting action, which)fjt
300 1784 1653 12 (consists of one of the following steps, following which the receive procedure exits.)fjt
300 1687 1776 14 (error: The packet is discarded. If the association was not configured by the initialization p)fjt
2076 1687 174 0 (rocedure)fjt
375 1627 1115 8 (\(peer.config bit not set\), the association is demobilized. )fjt
300 1521 1938 17 (recv: The packet procedure is called. If any of the sanity checks fail, proceed in the error step)fjt
2238 1521 13 0 (.)fjt
375 1461 1859 13 (Otherwise, the low-order bit of the reachability register \(peer.reach\) is set \(indicating the pee)fjt
2234 1461 17 0 (r)fjt
375 1401 813 6 (is reachable\) and the packet is discarded.)fjt
300 1295 1811 14 (xmit: The packet procedure is called \(to latch the packet variables\) and the packet discarded)fjt
2111 1295 139 1 (. Then,)fjt
375 1235 1837 14 (the poll-update procedure is called with argument peer.ppoll \(to insure the reply has the prop)fjt
2212 1235 39 0 (er)fjt
375 1175 1875 15 (value in the pkt.poll field\). Finally, the transmit procedure is called \(to send the packet and)fjt
375 1115 735 3 (possibly demobilize the association\).)fjt
300 1008 1950 17 (pkt: The packet procedure is called. If any of the sanity checks fail, proceed in the xmit step.)fjt
375 948 1859 13 (Otherwise, the low-order bit of the reachability register \(peer.reach\) is set \(indicating the pee)fjt
2234 948 17 0 (r)fjt
375 888 813 6 (is reachable\) and the packet is discarded.)fjt
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300 784 125 0 (3.4.3.)fjt
(e)19 (r)29 (u)30 (d)26 (e)27 (c)30 (o)18 (r)32 (P)13 ( )15 (t)27 (e)27 (k)27 (c)26 (a)33 (P)12 ( )0 17 425 784 fet
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300 679 1873 14 (The packet procedure checks the validity of the data, computes delay/offset samples and calls o)fjt
2173 679 77 0 (ther)fjt
300 621 1898 14 (procedures to select the peer and update the local clock. First, the following preliminary san)fjt
2198 621 53 0 (ity)fjt
300 563 442 2 (checks are performed:)fjt
300 457 38 0 (1.)fjt
375 457 1875 14 (The transmit timestamp \(pkt.xmt\) must not match the last one received from the same peer)fjt
375 399 1109 9 (\(peer.org\); if so, the message might be an old duplicate.)fjt
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308 2381 130 0 (Notes:)fjt
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383 2233 1860 15 (values. At other times the server simply broadcasts the local time with pkt.org and pkt.rec set)fjt
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383 2086 1860 13 (Ordinarily, these mode combinations would not be used; however, within the limits of the)fjt
383 2028 939 6 (specification, they would result in correct time.)fjt
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994 1922 563 4 (Table 6. Modes and Actions)fjt
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300 2340 38 0 (2.)fjt
375 2340 1875 15 (The originate timestamp \(pkt.org\) must match the last one sent to the same peer \(peer.xmt\); if)fjt
375 2282 1108 10 (not, the message might be out of order, bogus or worse.)fjt
300 2190 1802 18 (If either of these checks fail, a sanity flag is set which will be tested later. Before proceeding )fjt
2102 2190 148 0 (further,)fjt
300 2132 828 6 (the state variables are updated as follows:)fjt
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300 1424 1873 14 (At this point the poll-update procedure is called with argument peer.hpoll \(the peer poll inte)fjt
2173 1424 77 0 (rval)fjt
300 1365 1536 10 (\(peer.ppoll\) may have changed\). Then, the final sanity checks are performed:)fjt
300 1272 38 0 (3.)fjt
375 1272 1242 10 (The peer clock must be synchronized \(peer.leap not equal to 11)fjt
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1638 1272 612 6 (\) and the interval since the peer)fjt
375 1213 591 4 (clock was last updated satisfy)fjt
(,)30 (E)36 (G)36 (A)36 (X)36 (A)44 (M)13 (.)28 (P)30 (T)36 (N)13 ( )28 (<)12 ( )22 (e)39 (m)14 (i)13 (t)17 (f)22 (e)16 (r)13 (.)14 (t)25 (k)25 (p)12 ( )17 (-)12 ( )14 (t)39 (m)25 (x)12 (.)14 (t)25 (k)25 (p)0 35 872 1122 fet
375 1028 1432 10 (where NTP.MAXAGE is currently set to 86,400 seconds \(one full day\).)fjt
300 934 38 0 (4.)fjt
375 934 1775 15 (The peer.authentic bit must be set to one, either as the result of initial configuration \(recei)fjt
2150 934 101 1 (ve or)fjt
375 876 1011 5 (initialization procedures\) or the decrypt procedure.)fjt
300 782 38 0 (5.)fjt
375 782 1875 18 (If the peer.config bit is not set, the host must be able to synchronize to the peer; otherwise the)fjt
375 724 1693 11 (association will be demobilized; so, pkt.stratum must be not greater than sys.stratum.)fjt
300 632 1848 19 (If any of these checks fail, the sanity flag is set. If following all checks the sanity flag is se)fjt
2148 632 102 1 (t, the)fjt
(i)14 (t)15 ( )22 (c)22 (e)16 (r)13 (.)13 (t)25 (k)25 (p)15 ( )25 (d)25 (n)22 (a)15 ( )25 (g)16 (r)25 (o)13 (.)14 (t)25 (k)25 (p)14 ( )25 (h)14 (t)25 (o)25 (b)15 ( )13 (,)25 (n)25 (o)13 (i)14 (t)14 (i)25 (d)25 (d)22 (a)15 ( )25 (n)16 (I)15 ( )13 (.)19 (s)14 (t)14 (i)25 (x)22 (e)15 ( )22 (e)16 (r)25 (u)25 (d)22 (e)22 (c)25 (o)17 (r)25 (p)15 ( )13 (t)22 (e)25 (k)22 (c)22 (a)25 (p)15 ( )22 (e)25 (h)14 (t)15 ( )25 (d)25 (n)22 (a)15 ( )25 (d)22 (e)25 (d)17 (r)22 (a)22 (c)19 (s)14 (i)25 (d)15 ( )19 (s)14 (i)15 ( )22 (e)25 (g)22 (a)19 (s)19 (s)22 (e)39 (m)0 93 300 574 fet
(s)22 (e)38 (m)0 3 2154 574 fet
(-)0 1 2234 574 fet
300 516 1845 16 (tamps must be nonzero; if either is zero, the association has not synchronized or has lost reacha)fjt
2145 516 105 0 (bility)fjt
300 457 1811 18 (in one direction. In this case the packet procedure exits, but the sanity flag is not set. Note )fjt
2111 457 139 1 (that, in)fjt
(m)9 ( )14 (t)19 (s)22 (a)22 (c)25 (d)22 (a)25 (o)16 (r)25 (b)10 ( )25 (n)13 (i)10 ( )25 (g)25 (n)14 (i)13 (t)22 (a)17 (r)22 (e)25 (p)25 (o)9 ( )20 (s)13 (i)10 ( )16 (r)22 (e)22 (e)25 (p)10 ( )22 (e)25 (h)14 (t)9 ( )25 (d)25 (n)22 (a)10 ( )25 (o)16 (r)22 (e)22 (z)10 ( )25 (o)13 (t)10 ( )14 (l)22 (a)25 (u)25 (q)22 (e)9 ( )22 (e)17 (r)22 (a)9 ( )22 (c)22 (e)17 (r)12 (.)14 (t)25 (k)25 (p)9 ( )25 (d)25 (n)22 (a)10 ( )25 (g)16 (r)25 (o)13 (.)14 (t)25 (k)25 (p)9 ( )25 (h)14 (t)25 (o)25 (b)9 ( )22 (e)17 (r)22 (e)25 (h)36 (w)9 ( )22 (e)20 (s)22 (a)22 (c)9 ( )22 (e)25 (h)14 (t)0 98 300 399 fet
(e)25 (d)25 (o)0 3 2178 399 fet
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false true[1100 2880 1425 2880 ]arrowline stroke
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888 2425 774 5 (Figure 2. Calculating Delay and Offset)fjt
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%End page
showpage svobj restore gr
gs /svobj save def
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UserSoP
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300 2841 1842 16 (on a high-speed LAN, the timestamps can optionally be used as-is. If the host happens to synchr)fjt
2142 2841 108 0 (onize)fjt
300 2782 1828 14 (on the peer, the resulting systematic errors may be acceptable with or without further correc)fjt
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1075 2346 1534 2346 1534 2345 1075 2345 fa
gs eofill gr
1536 2327 25 1 ( .)fjt
300 2202 1950 13 (This method amounts to a continuously sampled, returnable-time system, which is used in some)fjt
300 2143 1884 15 (digital telephone networks [34]. Among the advantages are that the order and timing of the messa)fjt
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300 2085 1889 12 (is unimportant and that reliable delivery is not required. Obviously, the accuracies achieva)fjt
2189 2085 61 0 (ble)fjt
300 2027 1848 13 (depend upon the statistical properties of the outbound and inbound data paths. Further analysi)fjt
2148 2027 102 1 (s and)fjt
300 1969 1251 10 (experimental results bearing on this issue can be found in [44].)fjt
300 1875 1903 14 (In systems involving high-speed LANs, it may happen that, due to small differences in frequen)fjt
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300 1816 1144 10 (and precision between the host and peer clocks, the delay )fjt
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764 1512 1487 13 ( is still not greater than zero after this adjustment, the sample is discarded)fjt
300 1454 689 6 (and the packet procedure exits. If )fjt
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300 1395 1950 13 (value to prevent route flaps that can happen with Bellman-Ford algorithms and large delay)fjt
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(.)12 ( )17 (\))30 (T)28 (S)17 (I)36 (D)36 (N)16 (I)44 (M)13 (.)28 (P)30 (T)36 (N)13 ( )12 (,)0 16 1200 1243 fet
300 1149 1950 12 (Appropriate values for NTP.MAXSKW and NTP.MINDIST are given in Table 5. These values)fjt
300 1091 978 7 (may have to be altered for special circumstances.)fjt
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(l)22 (a)16 (r)25 (g)22 (e)14 (t)0 6 2137 822 fet
300 764 1950 15 (part of the NTP specification; however, one found to work well in the Internet environment is)fjt
300 706 1835 14 (described in Section 4. Finally, the clock-update procedure is called to reselect the clock sou)fjt
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300 647 750 5 (necessary, and update the local clock.)fjt
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300 457 1835 15 (When a primary clock is connected to the host, it is convenient to incorporate its informatio)fjt
2135 457 116 1 (n into)fjt
300 399 1915 18 (the data base as if the clock were represented as an ordinary peer. The clock can be polled once)fjt
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UserSoP
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300 2841 1950 17 (minute or so and the returned timecheck used to produce a new update for the local clock. The)fjt
300 2782 1772 13 (following peer variables can be established upon instantiation of the protocol machine for t)fjt
2072 2782 178 1 (he clock:)fjt
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(0)13 ( )0 2 1438 2403 fet
( )16 (r)25 (d)22 (a)22 (c)17 (r)19 (s)12 (.)17 (r)22 (e)22 (e)25 (p)0 12 1013 2344 fet
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(\254)0 1 1245 2344 fet
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(\))36 (w)25 (o)14 (l)22 (e)25 (b)13 ( )22 (e)22 (e)19 (s)16 (\()13 ( )0 12 1294 2344 fet
( )25 (y)22 (a)14 (l)22 (e)25 (d)14 (t)19 (s)22 (e)12 (.)17 (r)22 (e)22 (e)25 (p)0 14 1095 2286 fet
/tface 12 def
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(\254)0 1 1368 2286 fet
/tface 8 def
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(0)12 ( )0 2 1418 2286 fet
( )25 (p)19 (s)14 (i)25 (d)13 (t)20 (s)22 (e)12 (.)17 (r)22 (e)22 (e)25 (p)0 13 1107 2228 fet
/tface 12 def
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(\254)0 1 1356 2228 fet
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(0)13 ( )0 2 1405 2228 fet
300 2138 1895 14 (In this case the peer.distance and peer.srcadr can be constants reflecting the accuracy and type)fjt
2195 2138 56 1 ( of)fjt
300 2080 1850 13 (the clock, respectively. By convention, the value for peer.distance, which will become the val)fjt
2150 2080 100 1 (ue of)fjt
300 2021 1877 14 (sys.distance when the clock-update procedure is called, is ten times the expected mean error of)fjt
2177 2021 73 1 ( the)fjt
300 1963 1950 15 (clock, for instance, 100 milliseconds for a WWVB clock and 1000 milliseconds for a less accurate)fjt
300 1905 1950 15 (WWV clock. Also, the value for peer.srcadr, which will become the value of sys.refid when the)fjt
300 1847 1861 15 (clock-update procedure is called, is set to an ASCII string describing the clock type \(see Appe)fjt
2161 1847 89 0 (ndix)fjt
300 1788 65 0 (A\).)fjt
300 1698 1903 17 (When a valid timecode is received from the clock it is converted to internal timecheck form and t)fjt
2203 1698 47 0 (he)fjt
300 1640 467 2 (following variables set:)fjt
( )22 (c)22 (e)16 (r)13 (.)16 (r)22 (e)22 (e)25 (p)0 9 1057 1550 fet
/tface 12 def
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(\254)0 1 1228 1550 fet
/tface 8 def
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(k)22 (c)22 (e)25 (h)22 (c)22 (e)39 (m)14 (i)14 (t)12 ( )0 10 1277 1550 fet
( )14 (t)22 (e)19 (s)16 (f)17 (f)25 (o)14 (t)19 (s)22 (e)12 (.)17 (r)22 (e)22 (e)25 (p)0 15 890 1492 fet
/tface 12 def
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(\254)0 1 1169 1492 fet
/tface 8 def
/mpf true def
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(k)22 (c)25 (o)14 (l)22 (c)12 (.)20 (s)25 (y)19 (s)12 ( )17 (-)12 ( )25 (k)22 (c)22 (e)25 (h)22 (c)22 (e)39 (m)14 (i)14 (t)12 ( )0 22 1218 1492 fet
300 1402 1842 14 (Finally, the clock-update procedure is called to reselect the clock source, if necessary, and u)fjt
2142 1402 108 0 (pdate)fjt
300 1344 303 2 (the local clock.)fjt
300 1253 1903 15 (Since current broadcast time formats do not include advance notice of leap seconds, it may happ)fjt
2203 1253 47 0 (en)fjt
300 1195 1826 16 (that a leap second is correctly incorporated in the local timescale, but the radio clock may co)fjt
2126 1195 125 0 (ntinue)fjt
300 1137 1879 13 (at its old timescale until resynchronized. To avoid disruptions when a leap-second event occurs,)fjt
2179 1137 71 1 ( the)fjt
300 1079 1950 15 (clock should be disabled for some interval, depending on the clock type, until it has reliably)fjt
300 1020 775 4 (resynchronized to the broadcast signal.)fjt
300 930 1900 15 (With a little ingenuity the unused peer variables can be converted to control the clock polli)fjt
2200 930 50 0 (ng)fjt
300 872 1736 13 (interval, to determine its operating condition and even to use the clock-filter procedure i)fjt
2036 872 214 2 (n the usual)fjt
300 814 574 4 (way to improve its accuracy.)fjt
/tface 5 def
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300 723 125 0 (3.4.5.)fjt
(e)18 (r)30 (u)29 (d)27 (e)27 (c)29 (o)19 (r)29 (p)13 ( )27 (e)15 (t)27 (a)29 (d)30 (p)29 (u)16 (-)27 (k)26 (c)30 (o)13 (l)35 (C)12 ( )0 23 425 723 fet
/tface 8 def
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300 632 1950 14 (The clock-update procedure is called for a selected peer when a new delay/offset estimate is)fjt
300 574 1861 14 (available. First, the clock-selection procedure is called to determine the best peer on the bas)fjt
2161 574 90 1 (is of)fjt
300 516 1892 14 (estimated accuracy and reliability. If this results in a new clock source \(sys.peer\), the poll-upd)fjt
2192 516 58 0 (ate)fjt
300 457 1830 14 (procedure is called for sys.peer with argument peer.hpoll, since the poll interval for the new)fjt
2130 457 120 1 ( clock)fjt
300 399 1950 15 (source must be clamped at NTP.MINPOLL. If sys.hold is nonzero or sys.peer is not the selected)fjt
greset -300 3599 2850 3599 2850 -301 -300 -301 np mto lto lto lto clip np
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
2065 279 186 1 (Page 27)fjt
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300 2841 1825 14 (peer in the procedure call, the procedure exits. Otherwise, the state variables of the selecte)fjt
2125 2841 125 1 (d peer)fjt
300 2782 1109 9 (are used to update the system state variables as follows:)fjt
( )25 (p)22 (a)22 (e)14 (l)12 (.)20 (s)25 (y)19 (s)0 9 1068 2694 fet
/tface 12 def
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(\254)0 1 1239 2694 fet
/tface 8 def
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(p)22 (a)22 (e)14 (l)12 (.)17 (r)22 (e)22 (e)25 (p)12 ( )0 10 1289 2694 fet
( )39 (m)25 (u)14 (t)22 (a)16 (r)14 (t)19 (s)13 (.)19 (s)25 (y)19 (s)0 12 963 2636 fet
/tface 12 def
/mpf false def
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(\254)0 1 1200 2636 fet
/tface 8 def
/mpf true def
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(1)12 ( )28 (+)13 ( )39 (m)25 (u)13 (t)22 (a)17 (r)14 (t)19 (s)12 (.)17 (r)22 (e)22 (e)25 (p)12 ( )0 17 1250 2636 fet
( )22 (e)22 (c)25 (n)22 (a)14 (t)19 (s)14 (i)25 (d)13 (.)19 (s)25 (y)19 (s)0 13 819 2577 fet
/tface 12 def
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(\254)0 1 1070 2577 fet
/tface 8 def
/mpf true def
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(y)22 (a)13 (l)22 (e)25 (d)14 (t)19 (s)22 (e)13 (.)16 (r)22 (e)22 (e)25 (p)13 ( )12 ( )28 (+)13 ( )12 ( )22 (e)22 (c)25 (n)22 (a)14 (t)19 (s)14 (i)25 (d)13 (.)16 (r)22 (e)22 (e)25 (p)13 ( )0 32 1119 2577 fet
( )25 (n)25 (o)13 (i)20 (s)16 (r)22 (e)25 (p)19 (s)14 (i)25 (d)13 (.)19 (s)25 (y)19 (s)0 15 802 2519 fet
/tface 12 def
/mpf false def
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(\254)0 1 1095 2519 fet
/tface 8 def
/mpf true def
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(p)19 (s)14 (i)25 (d)14 (t)19 (s)22 (e)12 (.)17 (r)22 (e)22 (e)25 (p)12 ( )28 (+)13 ( )25 (n)25 (o)14 (i)19 (s)16 (r)22 (e)25 (p)20 (s)13 (i)25 (d)13 (.)16 (r)22 (e)22 (e)25 (p)13 ( )0 31 1144 2519 fet
( )25 (d)14 (i)16 (f)22 (e)17 (r)12 (.)19 (s)25 (y)20 (s)0 10 1043 2461 fet
/tface 12 def
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(\254)0 1 1226 2461 fet
/tface 8 def
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(r)25 (d)22 (a)22 (c)16 (r)20 (s)12 (.)17 (r)22 (e)22 (e)25 (p)12 ( )0 12 1275 2461 fet
( )22 (e)38 (m)14 (i)14 (t)16 (f)22 (e)17 (r)12 (.)20 (s)25 (y)19 (s)0 12 1049 2403 fet
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(\254)0 1 1281 2403 fet
/tface 8 def
/mpf true def
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(c)22 (e)16 (r)13 (.)16 (r)22 (e)22 (e)25 (p)13 ( )0 9 1330 2403 fet
300 2313 1749 13 (Finally, the local-clock procedure is called with peer.estoffset as argument to update the l)fjt
2049 2313 201 1 (ocal clock)fjt
300 2254 1885 17 (\(sys.clock\). It may happen that the local clock may be reset, rather than slewed to its final value)fjt
2185 2254 66 1 (. In)fjt
300 2196 1795 16 (this case the clear procedure is called repeatedly for every active peer to purge the clock filt)fjt
2095 2196 156 1 (er, reset)fjt
300 2138 1854 14 (the polling interval and reselect the clock source, if necessary. In addition, the system var)fjt
2154 2138 97 0 (iable)fjt
300 2080 329 3 (sys.hold is set to)fjt
( )25 (d)14 (l)25 (o)25 (h)13 (.)19 (s)25 (y)19 (s)0 9 761 1989 fet
/tface 12 def
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(\254)0 1 938 1989 fet
/tface 8 def
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( )31 (T)27 (F)17 (I)36 (H)28 (S)12 (.)33 (R)31 (E)30 (E)28 (P)13 ( )0 12 987 1989 fet
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(*)0 1 1285 1989 fet
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( )25 (1)16 (\()13 ( )0 4 1306 1989 fet
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( )24 (<)23 (<)0 3 1373 1989 fet
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(\))31 (L)30 (L)36 (O)28 (P)36 (N)16 (I)45 (M)12 (.)28 (P)30 (T)36 (N)13 ( )0 13 1432 1989 fet
300 1899 1936 14 (and subsequently decrements at one-second intervals to zero. This is necessary so that the loca)fjt
2236 1899 14 0 (l)fjt
300 1841 1805 16 (clock will not be updated until all clock filters fill up again and the dispersions settle down)fjt
2105 1841 13 0 (.)fjt
300 1751 1856 15 (Specification of the clock selection and local-clock algorithms is not an integral part of the )fjt
2156 1751 94 0 (NTP)fjt
300 1693 1950 13 (specification. A clock selection algorithm found to work well in the Internet environment is)fjt
300 1635 1873 15 (described in Section 4, while a local-clock algorithm is described in Section 5. The clock selec)fjt
2173 1635 78 0 (tion)fjt
300 1577 1867 15 (algorithm described in Section 4 usually picks the server at the lowest stratum and minimum d)fjt
2167 1577 83 0 (elay)fjt
300 1518 1889 16 (among all those available, unless that server appears to be a falseticker. The result is that )fjt
2189 1518 61 0 (the)fjt
300 1460 1900 14 (algorithms all work to build a minimum-weight spanning tree relative to the primary servers a)fjt
2200 1460 50 0 (nd)fjt
300 1402 1117 4 (thus a hierarchical-master-slave synchronization subnet.)fjt
300 1312 1950 18 (The basic NTP robustness model is that a host has no other means to verify time other than NTP)fjt
300 1253 1889 14 (itself. In some equipment a battery-backed clock/calendar is available for a sanity check. In )fjt
2189 1253 61 0 (the)fjt
300 1195 1794 13 (common assumption \(not always justified\) that the clock/calendar is more reliable, but less a)fjt
2094 1195 156 0 (ccurate,)fjt
300 1137 1899 15 (than the NTP synchronization subnet, the system clock can be set upon reboot by the clock/calend)fjt
2199 1137 51 0 (ar.)fjt
300 1079 1950 14 (Subsequent corrections can be determined by NTP as available, but only if the adjusted clock)fjt
300 1020 1950 14 (remains within a preconfigured range of the clock/calendar. When this is done an operator alarm)fjt
300 962 1474 13 (should be signalled if the adjustment is determined to be out of this range.)fjt
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300 871 125 0 (3.4.6.)fjt
(e)18 (r)29 (u)30 (d)27 (e)26 (c)30 (o)18 (r)32 (P)13 ( )30 (n)29 (o)13 (i)15 (t)27 (a)24 (z)13 (i)13 (l)26 (a)13 (i)16 (t)12 (i)30 (n)13 (I)12 ( )0 25 425 871 fet
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300 780 1365 10 (Upon reboot the NTP host initializes all system variables as follows:)fjt
( )25 (p)22 (a)22 (e)13 (l)13 (.)19 (s)25 (y)19 (s)0 9 943 690 fet
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(\254)0 1 1114 690 fet
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(1)25 (1)13 ( )0 3 1163 690 fet
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(2)0 1 1226 685 fet
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(\))25 (d)22 (e)22 (z)14 (i)25 (n)25 (o)16 (r)25 (h)22 (c)25 (n)25 (y)20 (s)25 (n)25 (u)16 (\()13 ( )0 17 1246 690 fet
( )39 (m)25 (u)14 (t)22 (a)16 (r)14 (t)19 (s)13 (.)19 (s)25 (y)19 (s)0 12 991 632 fet
/tface 12 def
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(\254)0 1 1228 632 fet
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(\))25 (d)22 (e)25 (n)14 (i)16 (f)22 (e)25 (d)25 (n)25 (u)17 (\()12 ( )25 (0)13 ( )0 14 1277 632 fet
( )25 (n)25 (o)13 (i)20 (s)13 (i)22 (c)22 (e)17 (r)25 (p)12 (.)20 (s)25 (y)19 (s)0 14 999 574 fet
/tface 12 def
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(\254)0 1 1270 574 fet
/tface 8 def
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(d)22 (e)16 (r)14 (i)25 (u)25 (q)22 (e)17 (r)12 ( )19 (s)22 (a)13 ( )0 12 1319 574 fet
( )22 (e)22 (c)25 (n)22 (a)13 (t)20 (s)13 (i)25 (d)13 (.)19 (s)25 (y)19 (s)0 13 984 516 fet
/tface 12 def
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(\254)0 1 1235 516 fet
/tface 8 def
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(\))25 (d)22 (e)25 (n)14 (i)16 (f)22 (e)25 (d)25 (n)25 (u)17 (\()12 ( )25 (0)13 ( )0 14 1284 516 fet
( )25 (n)25 (o)13 (i)20 (s)16 (r)22 (e)25 (p)19 (s)14 (i)25 (d)13 (.)19 (s)25 (y)19 (s)0 15 963 457 fet
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(\254)0 1 1256 457 fet
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(\))25 (d)22 (e)25 (n)14 (i)16 (f)22 (e)25 (d)25 (n)25 (u)17 (\()12 ( )25 (0)13 ( )0 14 1305 457 fet
( )25 (d)14 (i)16 (f)22 (e)17 (r)12 (.)20 (s)25 (y)19 (s)0 10 1018 399 fet
/tface 12 def
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(\254)0 1 1200 399 fet
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(\))25 (d)22 (e)25 (n)13 (i)17 (f)22 (e)25 (d)25 (n)25 (u)16 (\()13 ( )25 (0)12 ( )0 14 1250 399 fet
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300 2979 230 0 (RFC-1119)fjt
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( )22 (e)39 (m)14 (i)14 (t)16 (f)22 (e)17 (r)12 (.)19 (s)25 (y)20 (s)0 12 993 2841 fet
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(\))25 (d)22 (e)25 (n)14 (i)16 (f)22 (e)25 (d)25 (n)25 (u)17 (\()12 ( )25 (0)13 ( )0 14 1274 2841 fet
( )25 (k)22 (c)25 (o)14 (l)22 (c)12 (.)19 (s)25 (y)20 (s)0 10 921 2782 fet
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(\254)0 1 1118 2782 fet
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(e)14 (t)22 (a)39 (m)13 (i)14 (t)19 (s)22 (e)13 ( )22 (e)14 (l)25 (b)22 (a)13 (l)14 (i)22 (a)25 (v)22 (a)13 ( )13 (t)20 (s)22 (e)25 (b)12 ( )0 24 1167 2782 fet
( )25 (d)14 (l)25 (o)25 (h)12 (.)20 (s)25 (y)19 (s)0 9 1143 2724 fet
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(\254)0 1 1320 2724 fet
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(0)12 ( )0 2 1370 2724 fet
( )16 (r)22 (e)22 (e)25 (p)13 (.)19 (s)25 (y)19 (s)0 9 1091 2666 fet
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(\254)0 1 1265 2666 fet
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(L)31 (L)36 (U)36 (N)12 ( )0 5 1314 2666 fet
300 2571 1950 13 (The local clock \(sys.clock\) is presumably undefined at reboot; however, in some equipment an)fjt
300 2513 1903 12 (estimate is available from the reboot environment, such as a battery-backed clock/calendar. T)fjt
2203 2513 47 0 (he)fjt
300 2455 1812 12 (precision variable \(sys.precision\) is determined by the intrinsic architecture of the local ha)fjt
2112 2455 138 0 (rdware)fjt
300 2397 120 0 (clock.)fjt
300 2295 1779 12 (Next, an implementation-specific procedure is called repeatedly to mobilize a set of associ)fjt
2079 2295 171 1 (ations as)fjt
300 2236 1879 13 (required. The modes and addresses of these peers are determined using information read during)fjt
2179 2236 72 1 ( the)fjt
300 2178 1117 8 (reboot procedure or as the result of operator commands:)fjt
( )17 (r)25 (d)22 (a)22 (c)16 (r)20 (s)12 (.)17 (r)22 (e)22 (e)25 (p)0 12 976 2076 fet
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(\254)0 1 1208 2076 fet
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(s)19 (s)22 (e)16 (r)25 (d)25 (d)22 (a)13 ( )28 (P)16 (I)13 ( )16 (r)22 (e)22 (e)25 (p)13 ( )0 16 1257 2076 fet
( )14 (t)17 (r)25 (o)25 (p)22 (c)16 (r)19 (s)13 (.)16 (r)22 (e)22 (e)25 (p)0 13 975 2018 fet
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(\254)0 1 1223 2018 fet
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(t)17 (r)25 (o)25 (p)12 ( )28 (P)36 (D)36 (U)13 ( )16 (r)22 (e)22 (e)25 (p)13 ( )0 14 1272 2018 fet
( )16 (r)25 (d)22 (a)14 (t)19 (s)25 (d)13 (.)16 (r)22 (e)22 (e)25 (p)0 12 978 1960 fet
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(\254)0 1 1210 1960 fet
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(s)19 (s)22 (e)16 (r)25 (d)25 (d)22 (a)13 ( )28 (P)16 (I)13 ( )13 (t)20 (s)25 (o)25 (h)12 ( )0 16 1259 1960 fet
( )14 (t)16 (r)25 (o)25 (p)14 (t)19 (s)25 (d)13 (.)16 (r)22 (e)22 (e)25 (p)0 13 836 1901 fet
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(\254)0 1 1085 1901 fet
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(\))31 (T)33 (R)36 (O)28 (P)12 (.)28 (P)31 (T)36 (N)16 (\()13 ( )13 (t)17 (r)25 (o)25 (p)12 ( )28 (P)36 (D)36 (U)13 ( )13 (t)20 (s)25 (o)25 (h)12 ( )0 25 1134 1901 fet
( )25 (g)14 (i)16 (f)25 (n)25 (o)22 (c)13 (.)16 (r)22 (e)22 (e)25 (p)0 12 1113 1843 fet
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(\254)0 1 1351 1843 fet
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(1)12 ( )0 2 1400 1843 fet
( )22 (e)13 (l)25 (b)22 (a)25 (n)22 (e)25 (h)14 (t)25 (u)22 (a)13 (.)16 (r)22 (e)22 (e)25 (p)0 16 966 1785 fet
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(\254)0 1 1292 1785 fet
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(\))36 (w)25 (o)14 (l)22 (e)25 (b)13 ( )22 (e)22 (e)19 (s)16 (\()13 ( )0 12 1341 1785 fet
( )22 (c)14 (i)14 (t)25 (n)22 (e)25 (h)13 (t)25 (u)22 (a)13 (.)16 (r)22 (e)22 (e)25 (p)0 15 983 1727 fet
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(\))36 (w)25 (o)14 (l)22 (e)25 (b)12 ( )22 (e)22 (e)19 (s)17 (\()12 ( )0 12 1325 1727 fet
( )22 (e)25 (d)25 (o)39 (m)25 (h)13 (.)16 (r)22 (e)22 (e)25 (p)0 11 995 1668 fet
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(\254)0 1 1241 1668 fet
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(\))25 (d)22 (e)17 (r)13 (i)25 (u)25 (q)22 (e)17 (r)12 ( )20 (s)22 (a)16 (\()13 ( )0 14 1290 1668 fet
300 1566 1950 12 (If the optional authentication mechanism described in Appendix C is not implemented, the)fjt
300 1508 1931 15 (peer.authenable bit is set to zero and the peer.authentic bit ordinarily set to one, which allow)fjt
2231 1508 19 0 (s)fjt
300 1450 1837 15 (preconfigured peers to become the clock source. If peer.authentic bit is set to zero, a preconfi)fjt
2137 1450 114 0 (gured)fjt
300 1392 1934 15 (peer cannot become the clock source, regardless of stratum or mode. If the mechanism is imple)fjt
2234 1392 17 0 (-)fjt
300 1333 1741 13 (mented, additional variables are initialized as described in Appendix C. The values of thes)fjt
2041 1333 210 1 (e variables)fjt
(e)25 (h)14 (t)25 ( )22 (e)19 (s)22 (a)22 (c)25 ( )19 (s)14 (i)25 (h)14 (t)25 ( )25 (n)13 (i)25 ( )25 (y)14 (l)14 (i)16 (r)22 (a)25 (n)14 (i)25 (d)17 (r)36 (O)25 ( )12 (.)17 (f)13 (l)22 (e)20 (s)13 (t)14 (i)25 ( )28 (P)30 (T)36 (N)25 ( )17 (f)25 (o)25 ( )22 (e)25 (p)25 (o)23 (c)19 (s)25 ( )22 (e)26 (h)14 (t)25 ( )25 (d)25 (n)25 (o)26 (y)22 (e)25 (b)25 ( )20 (s)22 (e)17 (r)25 (u)25 (d)22 (e)23 (c)25 (o)17 (r)25 (p)25 ( )25 (g)25 (n)14 (i)20 (s)25 (u)25 ( )25 (d)23 (e)25 (n)14 (i)22 (a)14 (t)25 (b)26 (o)25 ( )22 (e)17 (r)22 (a)0 89 300 1275 fet
300 1217 1818 16 (peer.authenable bit is set to one and the peer.authentic bit to zero, so that only properly authen)fjt
2118 1217 132 0 (ticated)fjt
300 1159 702 5 (peers can become the clock source.)fjt
300 1057 1873 14 (FInally, the clear procedure is called to initialize the remaining peer variables and the ti)fjt
2173 1057 77 0 (mer)fjt
300 998 1468 9 (mechanism is armed and begins decrementing the peer timer \(peer.timer\).)fjt
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300 895 125 0 (3.4.7.)fjt
(e)18 (r)29 (u)30 (d)27 (e)26 (c)30 (o)18 (r)32 (P)13 ( )18 (r)27 (a)27 (e)13 (l)35 (C)12 ( )0 16 425 895 fet
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300 792 1950 15 (The clear procedure is called when some event occurs that results in a significant change in)fjt
300 734 1760 14 (reachability state or potential disruption of the local clock. The peer variables are updated a)fjt
2060 734 191 1 (s follows:)fjt
( )14 (l)13 (l)25 (o)25 (p)25 (h)13 (.)16 (r)22 (e)22 (e)25 (p)0 11 974 632 fet
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(\254)0 1 1187 632 fet
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(L)30 (L)36 (O)28 (P)36 (N)17 (I)44 (M)12 (.)28 (P)31 (T)36 (N)12 ( )0 12 1236 632 fet
( )25 (p)20 (s)13 (i)25 (d)14 (t)19 (s)22 (e)13 (.)16 (r)22 (e)22 (e)25 (p)0 13 941 574 fet
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(\254)0 1 1189 574 fet
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(P)28 (S)16 (I)36 (D)36 (X)36 (A)45 (M)12 (.)33 (R)31 (E)30 (E)28 (P)13 ( )0 13 1238 574 fet
( )16 (r)22 (e)14 (t)14 (l)13 (i)17 (f)12 (.)17 (r)22 (e)22 (e)25 (p)0 12 908 516 fet
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(\254)0 1 1115 516 fet
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(\))25 (d)22 (e)25 (n)14 (i)17 (f)22 (e)25 (d)25 (n)25 (u)12 ( )19 (s)22 (e)25 (g)22 (a)14 (t)19 (s)13 ( )14 (l)13 (l)22 (a)17 (\()12 ( )25 (0)13 ( )0 25 1164 516 fet
( )25 (d)14 (i)13 (l)22 (a)25 (v)13 (.)16 (r)22 (e)22 (e)25 (p)0 11 1127 457 fet
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(\254)0 1 1337 457 fet
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(0)12 ( )0 2 1386 457 fet
( )25 (g)17 (r)25 (o)12 (.)17 (r)22 (e)22 (e)25 (p)0 9 1143 399 fet
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(\254)0 1 1320 399 fet
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(0)13 ( )0 2 1369 399 fet
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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( )22 (c)22 (e)17 (r)12 (.)17 (r)22 (e)22 (e)25 (p)0 9 1146 2841 fet
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(\254)0 1 1317 2841 fet
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(0)13 ( )0 2 1366 2841 fet
( )14 (t)38 (m)25 (x)13 (.)16 (r)22 (e)22 (e)25 (p)0 9 1138 2782 fet
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(\254)0 1 1326 2782 fet
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(0)12 ( )0 2 1375 2782 fet
300 2693 1863 14 (Next, the poll-update procedure is called with argument peer.hpoll to reset the peer timer. Fin)fjt
2163 2693 87 0 (ally,)fjt
300 2635 1773 15 (the clock-selection procedure is called. If this results in a new clock source \(sys.peer\), the po)fjt
2073 2635 177 0 (ll-update)fjt
300 2576 1830 14 (procedure is called for sys.peer with argument peer.hpoll, since the poll interval for the new)fjt
2130 2576 120 1 ( clock)fjt
300 2518 894 6 (source must be clamped at NTP.MINPOLL. )fjt
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300 2425 125 0 (3.4.8.)fjt
(e)18 (r)29 (u)30 (d)27 (e)26 (c)30 (o)18 (r)30 (p)12 ( )27 (e)16 (t)26 (a)30 (d)29 (p)30 (u)15 (-)13 (l)13 (l)29 (o)33 (P)12 ( )0 22 425 2425 fet
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300 2332 1896 15 (The poll-update procedure is called when a significant event occurs that may result in a change)fjt
2196 2332 54 1 ( of)fjt
300 2274 1950 13 (the host-poll variable \(peer.hpoll\) or peer timer \(peer.timer\). The new value requested in the)fjt
300 2216 1950 13 (argument is first clamped to the range NTP.MINPOLL and NTP.MAXPOLL. If the peer happens)fjt
300 2158 1773 17 (to be the current clock source \(sys.peer\) or if the local clock is not crystal controlled, the n)fjt
2073 2158 177 1 (ew value)fjt
300 2099 1950 15 (is clamped to NTP.MINPOLL in order to avoid instabilities that can occur with larger values. Next,)fjt
300 2041 830 6 (compute the new poll interval in seconds:)fjt
( )25 (p)39 (m)22 (e)13 (t)0 5 456 1949 fet
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(=)0 1 568 1949 fet
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(1)13 ( )0 2 591 1949 fet
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(<)24 (<)12 ( )0 3 629 1949 fet
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(.)17 (])30 (L)31 (L)36 (O)27 (P)36 (N)17 (I)44 (M)13 (.)27 (P)31 (T)36 (N)12 ( )13 (,)16 (\))31 (L)30 (L)36 (O)28 (P)36 (X)36 (A)44 (M)13 (.)28 (P)30 (T)36 (N)13 ( )12 (,)14 (l)14 (l)25 (o)25 (p)25 (h)12 (.)17 (r)22 (e)22 (e)25 (p)12 ( )13 (,)13 (l)14 (l)25 (o)25 (p)25 (p)13 (.)16 (r)22 (e)22 (e)25 (p)17 (\()25 (n)13 (i)39 (m)17 ([)25 (x)22 (a)38 (m)13 ( )0 60 688 1949 fet
300 1857 1878 18 (If temp is equal to peer.timer, no change is necessary and the procedure exits. If temp is less t)fjt
2178 1857 72 0 (han)fjt
300 1799 1053 7 (peer.timer, peer.timer must be clamped to that value:)fjt
( )16 (r)22 (e)39 (m)14 (i)13 (t)13 (.)16 (r)22 (e)22 (e)25 (p)0 11 1081 1707 fet
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(\254)0 1 1296 1707 fet
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(.)25 (p)39 (m)22 (e)13 (t)13 ( )0 6 1345 1707 fet
300 1615 1834 16 (When the poll interval is changed and possibly large numbers of peers are involved, it is imp)fjt
2134 1615 116 0 (ortant)fjt
300 1557 1889 11 (to discourage tendencies to synchronize transmissions between the peers. A prudent preventat)fjt
2189 1557 61 0 (ive)fjt
300 1498 1468 11 (is to randomize the first transmission after the polling interval is changed:)fjt
( )17 (r)22 (e)39 (m)13 (i)14 (t)13 (.)16 (r)22 (e)22 (e)25 (p)0 11 958 1406 fet
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(\254)0 1 1173 1406 fet
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( )17 (r)22 (e)39 (m)13 (i)14 (t)13 (.)16 (r)22 (e)22 (e)25 (p)13 ( )0 12 1222 1406 fet
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( )21 (*)0 2 1450 1406 fet
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(,)17 (\))16 (\()25 (n)22 (a)17 (r)0 6 1483 1406 fet
( )25 (0)13 ( )13 (l)22 (a)25 (v)17 (r)22 (e)14 (t)25 (n)13 (i)13 ( )22 (e)25 (h)14 (t)12 ( )17 (r)22 (e)25 (v)25 (o)12 ( )19 (s)22 (e)25 (u)14 (l)22 (a)25 (v)13 ( )38 (m)25 (o)25 (d)25 (n)22 (a)17 (r)12 ( )20 (s)22 (e)22 (c)25 (u)25 (d)25 (o)16 (r)25 (p)13 ( )13 (t)22 (a)25 (h)14 (t)13 ( )25 (n)25 (o)13 (i)14 (t)22 (c)25 (n)25 (u)17 (f)12 ( )39 (m)22 (e)14 (t)19 (s)25 (y)19 (s)13 ( )22 (e)25 (h)13 (t)13 ( )19 (s)14 (i)12 ( )17 (\))16 (\()25 (n)22 (a)17 (r)12 ( )22 (e)17 (r)22 (e)25 (h)36 (w)0 83 300 1314 fet
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(\243)0 1 1950 1314 fet
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(.)25 (1)12 ( )28 (<)13 ( )25 (x)12 ( )0 7 1978 1314 fet
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300 1221 83 0 (3.5.)fjt
(s)27 (e)29 (u)27 (s)27 (s)13 (I)12 ( )12 (l)30 (o)18 (r)16 (t)29 (n)30 (o)35 (C)12 ( )27 (s)26 (s)27 (e)27 (c)27 (c)35 (A)12 ( )0 22 383 1221 fet
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300 1132 1900 13 (The NTP design is such that accidental or malicious data modification \(tampering\) or destructi)fjt
2200 1132 50 0 (on)fjt
300 1073 1950 15 (\(jamming\) at a time server should not in general result in timekeeping errors elsewhere in the)fjt
300 1015 1914 12 (synchronization subnet. However, the success of this approach depends on redundant time serve)fjt
2214 1015 36 0 (rs)fjt
300 957 1934 14 (and diverse network paths, together with the assumption that tampering or jamming will not occu)fjt
2234 957 17 0 (r)fjt
300 899 1839 15 (at many time servers throughout the synchronization subnet at the same time. In principle, the s)fjt
2139 899 111 0 (ubnet)fjt
300 840 1950 14 (vulnerability can be engineered through the selection of time servers known to be trusted and)fjt
300 782 1950 12 (allowing only those time servers to become the clock source. The authentication procedures)fjt
300 724 1950 12 (described in Appendix C represent one mechanism to enforce this; however, the encryption)fjt
300 666 1849 11 (algorithms can be quite CPU-intensive and can seriously degrade accuracy, unless precautions)fjt
2149 666 101 1 ( such)fjt
300 607 1331 10 (as mentioned in the description of the timeout procedure are taken.)fjt
300 516 1895 13 (While not a required feature of NTP itself, some implementations may include an access-cont)fjt
2195 516 55 0 (rol)fjt
300 457 1867 14 (feature that prevents unauthorized access and controls which peers are allowed to update the l)fjt
2167 457 83 0 (ocal)fjt
300 399 1803 15 (clock. For this purpose it is useful to distinguish between three categories of access: those )fjt
2103 399 148 1 (that are)fjt
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300 2841 1950 10 (preauthorized as trusted, preauthorized as friendly and all other \(non-preauthorized\) accesses.)fjt
300 2782 1909 13 (Presumably, preauthorization is accomplished by entries in the configuration file or some kind )fjt
2209 2782 42 0 (of)fjt
300 2724 1903 14 (ticket-management system such as Kerberos. In this model only trusted accesses can result in t)fjt
2203 2724 47 0 (he)fjt
300 2666 1903 14 (peer becoming the clock source. While friendly accesses cannot result in the peer becoming t)fjt
2203 2666 47 0 (he)fjt
300 2608 1402 9 (clock source, NTP messages and timestamps are returned as specified.)fjt
300 2513 1851 15 (It does not seem useful to maintain a secret clock, as would result from restricting non-preautho)fjt
2151 2513 99 0 (rized)fjt
300 2455 1757 14 (accesses, unless the intent is to hide the existence of the time server itself. Well-behave)fjt
2057 2455 194 1 (d Internet)fjt
300 2397 1950 14 (hosts should always return an ICMP error message if the service or resources are unavailable;)fjt
300 2338 1870 17 (however, in the case of NTP the resources required are minimal, so there is little need to res)fjt
2170 2338 80 0 (trict)fjt
300 2280 1878 14 (requests intended only to read the clock. A simple but effective access-control mechanism is t)fjt
2178 2280 72 0 (hen)fjt
300 2222 1934 16 (to consider all associations preconfigured in a symmetric mode or client mode \(modes 1, 2 and 3)fjt
2234 2222 17 0 (\))fjt
300 2164 1386 10 (as trusted and all other associations, preconfigured or not, as friendly.)fjt
300 2062 1903 16 (If a more comprehensive trust model is required, the design can be based on an access-control l)fjt
2203 2062 47 0 (ist)fjt
300 2003 1805 15 (with each entry consisting of a 32-bit Internet address, 32-bit mask and three-bit mode. If the)fjt
2105 2003 145 1 ( logical)fjt
300 1945 1931 15 (AND of the source address \(pkt.srcadr\) and the mask in an entry matches the corresponding addres)fjt
2231 1945 19 0 (s)fjt
300 1887 1895 17 (in the entry and the mode \(pkt.mode\) matches the mode in the entry, the access is allowed; otherw)fjt
2195 1887 55 0 (ise)fjt
300 1829 1950 15 (an ICMP error message is returned to the requestor. Through appropriate choice of mask, it is)fjt
300 1770 1819 14 (possible to restrict requests by mode to individual addresses, a particular subnet or net add)fjt
2119 1770 131 0 (resses,)fjt
300 1712 1789 15 (or have no restriction at all. The access-control list would then serve as a filter controllin)fjt
2089 1712 161 1 (g which)fjt
300 1654 624 3 (peers could create associations.)fjt
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300 1551 42 0 (4.)fjt
(s)43 (m)30 (h)15 (t)13 (i)18 (r)30 (o)29 (g)13 (l)35 (A)12 ( )30 (n)29 (o)13 (i)16 (t)26 (c)27 (e)13 (l)27 (e)32 (S)12 ( )30 (d)29 (n)27 (a)12 ( )30 (g)29 (n)13 (i)18 (r)27 (e)15 (t)13 (l)13 (i)29 (F)13 ( )0 35 350 1551 fet
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300 1456 1748 14 (A very important factor affecting the accuracy and reliability of time distribution is the c)fjt
2048 1456 203 1 (omplex of)fjt
300 1398 1873 16 (algorithms used to deglitch and smooth the offset estimates and to cast out outlyers due to fai)fjt
2173 1398 77 0 (lure)fjt
300 1339 1886 13 (of the primary reference sources or propagation media. The algorithms suggested in this sect)fjt
2186 1339 64 0 (ion)fjt
300 1281 1879 14 (were developed and refined over several years of operation in the Internet under widely varying)fjt
2179 1281 71 1 ( net)fjt
300 1223 1796 12 (configurations and utilizations. While these algorithms are believed the best available at the)fjt
2096 1223 154 1 ( present)fjt
300 1165 1928 15 (time, they are not an integral part of the NTP specification. A comprehensive discussion of th)fjt
2228 1165 22 0 (e)fjt
300 1106 1015 7 (design principles and performance is given in [44].)fjt
300 1004 1872 13 (There are two procedures described in the following, the clock-filter procedure, which is use)fjt
2172 1004 79 1 (d to)fjt
300 946 1884 15 (select the best offset samples from a given clock, and the clock-selection procedure, which is u)fjt
2184 946 66 0 (sed)fjt
300 888 1153 10 (to select the best clock among a hierarchical set of clocks.)fjt
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300 785 83 0 (4.1.)fjt
(e)18 (r)29 (u)30 (d)27 (e)26 (c)30 (o)18 (r)30 (p)12 ( )19 (r)26 (e)16 (t)13 (l)12 (i)16 (f)15 (-)27 (k)27 (c)29 (o)13 (l)35 (C)13 ( )0 23 383 785 fet
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300 690 1903 15 (The clock-filter procedure is executed upon arrival of an NTP message or other event that resu)fjt
2203 690 47 0 (lts)fjt
300 632 1721 13 (in new delay/offset sample pairs. The filter register \(peer.filter\) is initialized with zeros )fjt
2021 632 229 2 (by the clear)fjt
300 574 1864 15 (procedure. New sample pairs are shifted into peer.filter from the left end, causing first zeros )fjt
2164 574 86 0 (then)fjt
300 516 1803 17 (old sample pairs to shift off the right end. The transmit procedure will also shift zeros into pe)fjt
2103 516 147 0 (er.filter)fjt
300 457 1835 14 (when two polling intervals elapse without a fresh update. Then those sample pairs in peer.filte)fjt
2135 457 115 1 (r with)fjt
300 399 1903 16 (nonzero delay are inserted on a temporary list and sorted in order of increasing delay. The del)fjt
2203 399 47 0 (ay)fjt
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300 2841 1848 11 (estimate \(peer.estdelay\) and offset estimate \(peer.estoffset\) are chosen as the delay/offset v)fjt
2148 2841 102 0 (alues)fjt
300 2782 1756 13 (corresponding to the minimum-delay sample. In case of ties an arbitrary choice is made.)fjt
300 2686 1903 15 (The dispersion estimate \(peer.estdisp\) is then computed as the weighted sum of the offsets in t)fjt
2203 2686 47 0 (he)fjt
300 2628 466 5 (list. Assume the list has )fjt
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( )14 (t)19 (s)16 (r)14 (i)17 (f)10 ( )22 (e)25 (h)13 (t)11 ( )12 (,)19 (s)22 (e)14 (i)17 (r)13 (t)25 (n)22 (e)11 ( )30 (T)28 (F)16 (I)36 (H)28 (S)13 (.)33 (R)30 (E)31 (E)28 (P)10 ( )28 (=)10 ( )0 33 791 2628 fet
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(r)22 (e)25 (d)17 (r)25 (o)9 ( )25 (n)14 (i)9 ( )19 (s)22 (e)14 (l)25 (p)38 (m)22 (a)20 (s)9 ( )25 (d)13 (i)14 (l)22 (a)25 (v)9 ( )25 (n)14 (i)22 (a)14 (t)25 (n)25 (o)22 (c)9 ( )25 (h)22 (c)13 (i)25 (h)36 (w)9 ( )17 (f)25 (o)9 ( )0 40 1474 2628 fet
300 2570 449 4 (of increasing delay. If )fjt
/tface 10 def
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749 2570 31 0 (X)fjt
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780 2565 12 0 (i)fjt
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sf
( )25 (0)16 (\()13 ( )0 4 791 2570 fet
/tface 12 def
/mpf false def
sf
(\243)0 1 858 2570 fet
/tface 8 def
/mpf true def
sf
( )0 1 885 2570 fet
/tface 10 def
sf
( )14 (i)0 2 897 2570 fet
/tface 8 def
sf
( )28 (<)0 2 924 2570 fet
/tface 10 def
sf
(m)0 1 964 2570 fet
/tface 8 def
sf
( )22 (e)25 (h)14 (t)12 ( )17 (f)25 (o)12 ( )14 (t)22 (e)19 (s)17 (f)16 (f)25 (o)13 ( )22 (e)25 (h)13 (t)13 ( )19 (s)14 (i)12 ( )17 (\))0 23 1000 2570 fet
/tface 10 def
sf
(i)0 1 1401 2570 fet
/tface 8 def
sf
(s)22 (e)25 (u)14 (l)22 (a)25 (v)13 ( )22 (e)25 (h)13 (t)13 ( )22 (e)14 (t)25 (u)25 (p)38 (m)25 (o)22 (c)13 ( )25 (n)22 (e)25 (h)14 (t)12 ( )13 (,)22 (e)13 (l)25 (p)39 (m)22 (a)19 (s)13 ( )25 (h)14 (t)0 34 1414 2570 fet
/tface 10 def
sf
700 2465 25 0 (d)fjt
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
sf
725 2460 12 0 (i)fjt
/tface 8 def
sf
( )0 1 737 2460 fet
( )24 (=)0 2 749 2465 fet
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
sf
( )10 (|)0 2 785 2465 fet
/tface 10 def
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(X)0 1 808 2465 fet
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
sf
(i)0 1 838 2460 fet
/tface 8 def
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
sf
( )0 1 850 2465 fet
/tface 12 def
/mpf false def
sf
(-)0 1 862 2465 fet
/tface 8 def
/mpf true def
sf
( )0 1 889 2465 fet
/tface 10 def
sf
(X)0 1 902 2465 fet
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
sf
(0)0 1 932 2460 fet
/tface 8 def
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
sf
(f)14 (i)12 ( )10 (|)13 ( )0 5 953 2465 fet
/tface 10 def
sf
(i)13 ( )0 2 1018 2465 fet
/tface 8 def
sf
( )0 1 1045 2465 fet
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
sf
( )24 (<)0 2 1057 2465 fet
/tface 10 def
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(m)0 1 1093 2465 fet
/tface 8 def
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( )10 (|)12 ( )25 (d)25 (n)22 (a)13 ( )0 7 1129 2465 fet
/tface 10 def
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(X)0 1 1249 2465 fet
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
sf
(i)0 1 1279 2460 fet
/tface 8 def
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
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( )0 1 1291 2465 fet
/tface 12 def
/mpf false def
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( )27 (-)13 ( )0 3 1303 2465 fet
/tface 10 def
/mpf true def
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(X)0 1 1355 2465 fet
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( )21 (0)0 2 1386 2460 fet
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( )10 (|)0 2 1419 2465 fet
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
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( )23 (<)0 2 1442 2465 fet
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
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(;)27 (P)28 (S)17 (I)36 (D)36 (X)36 (A)44 (M)12 (.)34 (R)30 (E)31 (E)27 (P)0 13 1478 2465 fet
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944 2407 25 0 (d)fjt
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969 2402 12 0 (i)fjt
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(=)12 ( )0 2 981 2407 fet
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
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(,)22 (e)20 (s)13 (i)36 (w)17 (r)22 (e)25 (h)14 (t)25 (o)12 ( )28 (P)28 (S)16 (I)36 (D)36 (X)36 (A)44 (M)13 (.)33 (R)31 (E)30 (E)28 (P)12 ( )0 24 1017 2407 fet
( )25 (p)19 (s)14 (i)25 (d)14 (t)19 (s)22 (e)13 (.)16 (r)22 (e)22 (e)25 (p)0 13 1047 2239 fet
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
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(=)0 1 1295 2239 fet
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
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( )0 1 1319 2239 fet
/tface 12 def
/mpf false def
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sf
(\345)0 1 1331 2227 fet
/tface 8 def
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( )0 1 1385 2239 fet
/tface 10 def
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(i)0 1 1320 2175 fet
/tface 8 def
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( )0 1 1332 2175 fet
/tface 12 def
/mpf false def
sf
(=)0 1 1342 2175 fet
/tface 8 def
/mpf true def
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( )0 1 1365 2175 fet
(0)0 1 1375 2175 fet
/tface 10 def
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(n)0 1 1316 2310 fet
/tface 8 def
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( )0 1 1336 2310 fet
/tface 12 def
/mpf false def
sf
(-)0 1 1347 2310 fet
/tface 8 def
/mpf true def
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( )0 1 1369 2310 fet
(1)0 1 1380 2310 fet
/tface 10 def
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(d)0 1 1397 2239 fet
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(i)0 1 1422 2234 fet
/tface 8 def
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( )0 1 1434 2239 fet
/tface 10 def
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(w)0 1 1446 2239 fet
/tface 8 def
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( )0 1 1479 2239 fet
/tface 10 def
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
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(i)0 1 1492 2263 fet
/tface 8 def
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300 2072 136 1 (where )fjt
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436 2072 33 0 (w)fjt
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( )25 (o)13 (t)14 ( )14 (t)22 (e)19 (s)14 ( )25 (y)14 (l)14 (t)25 (n)22 (e)16 (r)17 (r)25 (u)22 (c)14 ( )12 (,)17 (\))16 (r)22 (e)14 (t)22 (e)39 (m)22 (a)16 (r)22 (a)25 (p)14 ( )34 (R)30 (E)31 (T)30 (L)17 (I)27 (F)13 (.)33 (R)31 (E)30 (E)28 (P)14 ( )22 (e)25 (h)14 (t)14 ( )25 (d)22 (e)13 (l)14 (l)22 (a)22 (c)14 ( )25 (o)19 (s)14 (l)22 (a)17 (\()14 ( )16 (r)25 (o)14 (t)22 (c)22 (a)16 (f)14 ( )25 (g)25 (n)14 (i)14 (t)25 (h)25 (g)14 (i)22 (e)36 (w)14 ( )22 (a)14 ( )19 (s)14 (i)14 ( )25 (1)14 ( )28 (<)14 ( )0 85 469 2072 fet
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(1)0 1 2182 2096 fet
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(/)0 1 2203 2072 fet
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(2)0 1 2217 2067 fet
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(,)0 1 2238 2072 fet
300 2014 1766 10 (experimentally adjusted to match typical offset distributions. The peer.estdisp variable is)fjt
2066 2014 184 1 ( intended)fjt
300 1956 1864 14 (for use as a quality indicator, with increasing values associated with decreasing quality and g)fjt
2164 1956 86 0 (iven)fjt
300 1897 830 6 (less weight in the clock selection process.)fjt
300 1793 1934 14 (The peer.estdisp variable is used in the transmit procedure to determine whether to increase o)fjt
2234 1793 17 0 (r)fjt
300 1735 1950 12 (decrease the polling interval. If peer.estdisp is greater than the PEER.THRESHOLD parameter, the)fjt
300 1676 1792 13 (path quality is deteriorating and the polling interval is decreased; otherwise, the polling in)fjt
2092 1676 159 1 (terval is)fjt
300 1618 1886 12 (increased. The peer.estdisp variable is also used in the clock-selection procedure in conjunct)fjt
2186 1618 64 0 (ion)fjt
300 1560 1870 13 (with the peer.dispersion variable as a means to select candidate peers for clock synchronizatio)fjt
2170 1560 38 0 (n.)fjt
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300 1454 83 0 (4.2.)fjt
(e)18 (r)30 (u)29 (d)27 (e)27 (c)29 (o)19 (r)29 (p)13 ( )29 (n)30 (o)13 (i)15 (t)27 (c)27 (e)12 (l)27 (e)27 (s)15 (-)27 (k)27 (c)29 (o)13 (l)35 (C)13 ( )0 26 383 1454 fet
/tface 8 def
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300 1358 1847 10 (The clock-selection procedure uses the values of delay \(peer.estdelay\), offset \(peer.estoffset)fjt
2147 1358 103 1 (\) and)fjt
300 1299 1803 12 (dispersion \(peer.estdisp\) calculated by the clock-filter procedure for each peer and is calle)fjt
2103 1299 147 1 (d when)fjt
300 1241 1838 15 (these values change or when the reachability status changes. It constructs a list of candidate se)fjt
2138 1241 112 0 (rvers,)fjt
300 1183 1854 17 (then sorts it in order of estimated robustness and trims it to manageable size. Next, it sorts th)fjt
2154 1183 96 1 (e list)fjt
300 1125 1810 15 (in order of estimated accuracy and repeatedly casts out outlyers on the basis of dispersion un)fjt
2110 1125 140 1 (til only)fjt
300 1066 1757 15 (the most reliable, most accurate candidates are left. The only output from this procedure is t)fjt
2057 1066 193 1 (he system)fjt
300 1008 1783 18 (variable sys.peer, which is set as a pointer to a surviving candidate, if there is one, or to th)fjt
2083 1008 167 1 (e NULL)fjt
300 950 239 2 (value if not.)fjt
300 845 1878 14 (The first subprocedure begins by constructing a list of candidates sorted first by peer.stratum )fjt
2178 845 72 0 (and)fjt
300 787 1082 8 (then by the sum of peer.estdisp, peer.dispersion and )fjt
/tface 12 def
/mpf false def
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1382 787 25 0 (d)fjt
/tface 8 def
/mpf true def
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1407 787 843 7 ( \(see Section 3.4.3\). The only criteria for)fjt
300 729 1417 12 (membership on this list is that the peer must pass certain sanity checks:)fjt
300 622 38 0 (1.)fjt
375 622 1875 15 (The variable peer.stratum must be greater than zero and less than the maximum that can be)fjt
375 564 867 5 (encoded as a packet variable \(NTP.INFIN\).)fjt
300 457 38 0 (2.)fjt
375 457 1875 13 (If peer.stratum is greater than one \(synchronized by NTP\), peer.refid must not match peer.dstadr;)fjt
375 399 1038 6 (otherwise, a synchronization loop would be formed.)fjt
greset -300 3599 2850 3599 2850 -301 -300 -301 np mto lto lto lto clip np
greset -75 3374 2624 3374 2624 2825 -75 2825 np mto lto lto lto clip np
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
greset -300 3599 2850 3599 2850 -301 -300 -301 np mto lto lto lto clip np
greset -75 474 2624 474 2624 -75 -75 -75 np mto lto lto lto clip np
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300 279 100 0 (Mills)fjt
2065 279 186 1 (Page 32)fjt
greset -300 3599 2850 3599 2850 -301 -300 -301 np mto lto lto lto clip np
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UserSoP
greset -75 3374 2624 3374 2624 -75 -75 -75 np mto lto lto lto clip np
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300 2839 38 0 (3.)fjt
375 2839 1875 11 (Both peer.estdelay and peer.estdisp must be less than NTP.MAXWGT \(currently 8\), which)fjt
375 2781 1875 16 (insures that the filter register is at least half full, yet avoids using data from very noisy)fjt
375 2722 802 3 (associations or broken implementations.)fjt
300 2627 1853 16 (If no candidates survive the above sanity checks, the current clock source \(sys.peer\) is set to N)fjt
2153 2627 97 0 (ULL)fjt
300 2569 789 4 (and the clock-selection procedure exits.)fjt
300 2474 1950 17 (The list is then pruned from the end to be no longer than the maximum select size parameter)fjt
300 2415 1950 14 (\(NTP.MAXLIST\), currently set to five; however, the current clock source is not pruned from the)fjt
300 2357 1950 12 (list, regardless of position. This feature minimizes clock wander \(see below\) on high-speed,)fjt
(t)25 (n)22 (e)13 ( )13 (t)20 (s)16 (r)14 (i)16 (f)13 ( )22 (e)25 (h)14 (t)13 ( )14 (t)22 (a)13 ( )25 (d)22 (e)14 (t)22 (a)22 (c)25 (n)25 (u)16 (r)14 (t)13 ( )20 (s)13 (i)14 ( )13 (t)20 (s)13 (i)14 (l)13 ( )22 (e)25 (h)14 (t)13 ( )25 (g)25 (n)14 (i)25 (n)25 (n)14 (i)25 (g)22 (e)25 (b)13 ( )22 (e)25 (h)14 (t)13 ( )39 (m)25 (o)16 (r)17 (f)13 ( )25 (g)25 (n)14 (i)14 (t)16 (r)22 (a)14 (t)28 (S)13 ( )13 ( )13 (.)19 (s)25 (k)16 (r)25 (o)36 (w)14 (t)22 (e)25 (n)13 ( )17 (r)22 (e)25 (v)16 (r)22 (e)20 (s)16 (-)22 (e)14 (l)25 (p)14 (i)13 (t)14 (l)25 (u)39 (m)0 93 300 2299 fet
(e)16 (r)22 (e)25 (h)36 (w)13 ( )25 (y)17 (r)0 8 2074 2299 fet
(r)24 (e)15 (t)23 (e)40 (m)23 (a)18 (r)23 (a)26 (p)25 ( )24 (a)15 (t)23 (a)18 (r)15 (t)20 (s)25 ( )15 (t)23 (c)24 (e)15 (l)23 (e)20 (s)25 ( )40 (m)27 (u)40 (m)15 (i)26 (x)23 (a)40 (m)25 ( )23 (e)27 (h)15 (t)25 ( )20 (s)26 (d)24 (e)23 (e)23 (c)26 (x)24 (e)25 ( )15 (t)20 (s)15 (i)15 (l)25 ( )23 (e)27 (h)15 (t)25 ( )26 (n)15 (i)25 ( )23 (a)15 (t)23 (a)18 (r)15 (t)21 (s)25 ( )15 (t)26 (n)23 (e)18 (r)23 (e)18 (f)18 (f)15 (i)27 (d)25 ( )18 (f)26 (o)25 ( )18 (r)24 (e)26 (b)41 (m)26 (u)27 (n)25 ( )23 (e)27 (h)15 (t)0 86 300 2241 fet
(e)14 (l)25 (b)22 (a)14 (i)14 (l)23 (e)16 (r)25 ( )23 (e)22 (c)25 (u)25 (d)26 (o)16 (r)26 (p)25 ( )25 (o)14 (t)25 ( )25 (d)25 (n)26 (u)25 (o)17 (f)25 ( )25 (n)22 (e)22 (e)26 (b)25 ( )22 (e)25 (v)22 (a)26 (h)25 ( )19 (s)22 (e)14 (l)26 (u)16 (r)25 ( )23 (e)19 (s)22 (e)26 (h)30 (T)25 ( )13 (.)26 (o)36 (w)15 (t)25 ( )25 (o)14 (t)25 ( )15 (t)22 (e)20 (s)25 ( )25 (y)15 (l)14 (t)25 (n)23 (e)17 (r)17 (r)25 (u)23 (c)25 ( )13 (,)17 (\))36 (A)34 (R)31 (T)28 (S)37 (X)36 (A)45 (M)13 (.)28 (P)31 (T)37 (N)17 (\()0 84 300 2182 fet
300 2124 1873 12 (synchronization candidates over a wide range of system environments while minimizing the pul)fjt
2173 2124 78 0 (ling)fjt
300 2066 1827 12 (effect of high-stratum, high-dispersion peers, especially when large numbers of peers are inv)fjt
2127 2066 123 0 (olved.)fjt
300 1971 1898 13 (The next subprocedure is designed to detect falsetickers or other conditions which might result)fjt
2198 1971 52 1 ( in)fjt
300 1912 1831 13 (gross errors. This is done by repeatedly casting out outlyers which exhibit significant dispe)fjt
2131 1912 119 0 (rsions)fjt
300 1854 1815 13 (relative to the other members of the list. However, indiscriminately casting out outlyers be)fjt
2115 1854 136 1 (yond a)fjt
300 1796 1892 16 (limit set by the intrinsic precisions of the local clocks can result in wandering among the serv)fjt
2192 1796 58 0 (ers)fjt
300 1738 1903 10 (without producing meaningful improvements in reliability or accuracy, especially on high-spe)fjt
2203 1738 47 0 (ed)fjt
300 1679 1846 13 (LANs using several redundant time servers with similar delays, offsets and dispersions. In add)fjt
2146 1679 104 0 (ition,)fjt
300 1621 1950 12 (wandering causes needless network overhead, since the poll interval is clamped at NTP.MINPOLL)fjt
300 1563 1753 14 (for each server selected and only slowly increases when the server is no longer selected.)fjt
300 1467 1950 11 (The subprocedure is designed to strike a balance between falseticker discrimination, accuracy)fjt
300 1409 1950 14 (optimization and wander avoidance. First, the candidate list is resorted in the order first by)fjt
300 1351 859 6 (peer.stratum and then by peer.estdelay. Let )fjt
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1159 1351 36 0 (m)fjt
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1195 1351 1055 10 ( be the number of candidates remaining in the list and)fjt
300 1293 148 2 (for the )fjt
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448 1293 14 0 (i)fjt
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462 1293 323 3 (th candidate let )fjt
/tface 10 def
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785 1293 31 0 (X)fjt
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816 1288 12 0 (i)fjt
/tface 8 def
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827 1288 15 1 ( )fjt
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842 1293 684 6 (be the value of peer.estoffset and )fjt
/tface 12 def
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1525 1293 25 0 (d)fjt
/tface 10 def
/mpf true def
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1550 1288 12 0 (i)fjt
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1561 1293 689 6 ( as described in Section 3.4.3. For)fjt
300 1234 104 1 (each )fjt
/tface 10 def
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404 1234 14 0 (i)fjt
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( )25 (0)16 (\()13 ( )0 4 417 1234 fet
/tface 12 def
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(\243)0 1 484 1234 fet
/tface 8 def
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( )0 1 511 1234 fet
/tface 10 def
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(i)0 1 524 1234 fet
/tface 8 def
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( )28 (<)13 ( )0 3 537 1234 fet
/tface 10 def
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(m)0 1 590 1234 fet
/tface 8 def
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( )25 (n)25 (o)14 (i)19 (s)16 (r)22 (e)25 (p)20 (s)13 (i)25 (d)13 ( )22 (e)25 (h)14 (t)12 ( )22 (e)14 (t)25 (u)25 (p)39 (m)25 (o)22 (c)12 ( )17 (\))0 25 626 1234 fet
/tface 10 def
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(d)0 1 1130 1234 fet
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(i)0 1 1155 1229 fet
/tface 8 def
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( )25 (o)14 (t)12 ( )22 (e)25 (v)14 (i)14 (t)22 (a)14 (l)22 (e)16 (r)13 ( )13 (t)20 (s)13 (i)14 (l)13 ( )22 (e)25 (h)13 (t)13 ( )16 (f)25 (o)13 ( )0 25 1166 1234 fet
/tface 10 def
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(i)0 1 1591 1234 fet
/tface 8 def
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(:)0 1 1605 1234 fet
/tface 10 def
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(d)0 1 1065 1077 fet
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
sf
(i)0 1 1090 1071 fet
/tface 8 def
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
sf
( )0 1 1102 1077 fet
/tface 12 def
/mpf false def
sf
(=)0 1 1114 1077 fet
/tface 8 def
/mpf true def
sf
( )0 1 1142 1077 fet
/tface 12 def
/mpf false def
/txscale 1800 3 mul 72 div def /tyscale 1800 3 mul 72 div def
sf
(\345)0 1 1154 1068 fet
/tface 8 def
/mpf true def
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
sf
( )0 1 1207 1077 fet
/tface 10 def
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
sf
(j)0 1 1143 1012 fet
/tface 8 def
sf
( )0 1 1154 1012 fet
/tface 12 def
/mpf false def
sf
(=)0 1 1165 1012 fet
/tface 8 def
/mpf true def
sf
( )0 1 1187 1012 fet
(0)0 1 1198 1012 fet
/tface 10 def
sf
(m)0 1 1134 1147 fet
/tface 8 def
sf
( )0 1 1164 1147 fet
/tface 12 def
/mpf false def
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(-)0 1 1174 1147 fet
/tface 8 def
/mpf true def
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( )0 1 1197 1147 fet
(1)0 1 1207 1147 fet
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
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(|)0 1 1220 1077 fet
( )0 1 1230 1077 fet
/tface 10 def
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(X)0 1 1242 1077 fet
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
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(i)0 1 1273 1071 fet
/tface 8 def
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
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( )0 1 1284 1077 fet
/tface 12 def
/mpf false def
sf
(-)0 1 1297 1077 fet
/tface 8 def
/mpf true def
sf
( )0 1 1324 1077 fet
/tface 10 def
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(X)0 1 1337 1077 fet
/txscale 1000 3 mul 72 div def /tyscale 1000 3 mul 72 div def
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(j)0 1 1367 1071 fet
/tface 8 def
/txscale 1200 3 mul 72 div def /tyscale 1200 3 mul 72 div def
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( )0 1 1379 1077 fet
(|)0 1 1391 1077 fet
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435 919 22 0 (v)fjt
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(\))16 (])25 (4)25 (4)17 ([)13 ( )22 (e)22 (e)19 (s)17 (\()13 ( )22 (c)14 (i)14 (t)19 (s)14 (i)16 (r)22 (e)14 (t)22 (c)22 (a)16 (r)22 (a)25 (h)22 (c)14 ( )25 (d)22 (e)16 (r)14 (i)19 (s)22 (e)25 (d)13 ( )22 (e)25 (h)14 (t)13 ( )17 (r)25 (o)16 (f)14 ( )25 (d)22 (e)13 (t)20 (s)25 (u)13 (j)25 (d)22 (a)14 ( )25 (y)14 (l)13 (l)22 (a)14 (t)25 (n)22 (e)39 (m)14 (i)16 (r)22 (e)25 (p)25 (x)22 (e)14 ( )16 (r)25 (o)14 (t)22 (c)22 (a)16 (f)14 ( )25 (g)25 (n)14 (i)13 (t)25 (h)25 (g)14 (i)22 (e)36 (w)14 ( )22 (a)13 ( )19 (s)14 (i)14 ( )25 (1)13 ( )28 (<)14 ( )0 92 457 919 fet
(.)0 1 2238 919 fet
300 861 205 2 (Note that )fjt
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530 856 12 0 (i)fjt
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541 861 805 7 ( represents the actual dispersion of the )fjt
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1360 861 598 5 (th candidate, while the skew )fjt
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1994 861 256 2 ( represents a)fjt
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300 516 886 7 (then cast out the candidate with maximum )fjt
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1211 511 12 0 (i)fjt
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1222 516 698 8 ( or, in case of ties, the maximum )fjt
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1920 516 14 0 (i)fjt
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1933 516 317 3 (, and repeat the)fjt
300 457 1950 13 (subprocedure; otherwise, stop. If the current clock source \(sys.peer\) is one of the surviving)fjt
300 399 1876 16 (candidates in the list and there is no other surviving candidate of lower stratum, then simply )fjt
2176 399 75 0 (exit)fjt
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1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
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300 2157 1831 11 (the clock-selection procedure without doing anything further. Otherwise, set sys.peer to point)fjt
2131 2157 120 2 ( to the)fjt
300 2099 1262 9 (first candidate in the list and exit the clock-selection procedure.)fjt
300 2006 1928 17 (This subprocedure is designed to favor those candidates near the head of the list, which are at th)fjt
2228 2006 22 0 (e)fjt
300 1948 1950 14 (lowest stratum and lowest delay and presumably can provide the most precise time. With proper)fjt
300 1890 609 4 (selection of weighting factor )fjt
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909 1890 22 0 (v)fjt
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931 1890 939 7 ( \(also called NTP.SELECT\), currently set to )fjt
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1870 1914 21 0 (3)fjt
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1890 1890 14 0 (/)fjt
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1904 1885 21 0 (4)fjt
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1925 1890 325 3 (, entries will be)fjt
300 1831 1873 15 (trimmed from the tail of the list, unless a few outlyers disagree significantly with respect to)fjt
2173 1831 77 1 ( the)fjt
300 1773 1917 13 (remaining entries, in which case the outlyers are discarded first. The termination condition )fjt
2217 1773 33 0 (is)fjt
300 1715 1872 11 (designed to avoid needless switching between clock sources when not statistically justified,)fjt
2172 1715 78 1 ( yet)fjt
300 1657 1950 13 (maintain a bias toward the low-stratum, low-delay peers. In some situations, such as multiple)fjt
300 1598 1886 14 (broadcast servers on a high-capacity LAN, it may be useful to fine-tune the termination condit)fjt
2186 1598 64 0 (ion)fjt
300 1540 1793 18 (even more, such as always permitting a switch to the first candidate in the list if that peer is)fjt
2093 1540 158 1 ( already)fjt
300 1482 1032 6 (operating at a host-poll interval of NTP.MINPOLL.)fjt
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300 1383 42 0 (5.)fjt
(s)27 (k)27 (c)29 (o)13 (l)35 (C)13 ( )13 (l)26 (a)27 (c)30 (o)29 (L)13 ( )0 13 350 1383 fet
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300 1290 1912 17 (In order to implement a precise and accurate local clock, the host must be equipped with a hardwa)fjt
2212 1290 39 0 (re)fjt
300 1232 1785 14 (clock consisting of an oscillator and interface and capable of the required precision and sta)fjt
2085 1232 165 1 (bility. A)fjt
300 1173 1903 13 (logical clock is then constructed using these components plus software components that adjust t)fjt
2203 1173 47 0 (he)fjt
300 1115 1934 14 (apparent time and frequency in response to periodic corrections computed by NTP or some othe)fjt
2234 1115 17 0 (r)fjt
300 1057 1950 13 (time-synchronization protocol such as Hellospeak [21] or the Unix 4.3bsd TSP [26]. This section)fjt
300 999 1851 13 (includes a summary of the characteristics of various standard oscillators, followed by the m)fjt
2151 999 83 0 (athe)fjt
2234 999 17 0 (-)fjt
300 940 1950 14 (matical model of a method to synchronize an oscillator to a reference standard. The section)fjt
300 882 1950 11 (concludes with a description of the Fuzzball logical-clock model and implementation, which)fjt
300 824 1928 14 (includes provisions for precise time and frequency adjustment and can maintain time to within )fjt
2228 824 22 0 (a)fjt
300 766 1171 8 (millisecond and frequency to within a millisecond per day.)fjt
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300 667 83 0 (5.1.)fjt
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300 574 1812 13 (As mentioned previously, a primary frequency standard is an oscillator that can maintain ext)fjt
2112 574 138 0 (remely)fjt
300 516 1782 14 (precise frequency relative to a physical phenomenon, such as a transition in the orbital sta)fjt
2082 516 168 2 (tes of an)fjt
300 457 1799 12 (electron. Presently available atomic oscillators are based on the transitions of the hydrogen,)fjt
2099 457 152 1 ( cesium)fjt
300 399 557 4 (and rubidium atoms. Table 7)fjt
857 399 1393 10 ( shows the characteristics for typical oscillators of these types compared)fjt
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gs eofill gr
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gs eofill gr
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gs eofill gr
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476 2838 294 1 (Oscillator type)fjt
1094 2838 362 2 (Stability \(per day\))fjt
1712 2838 340 2 (Drift /Aging \(per)fjt
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476 2708 331 1 (Hydrogen maser)fjt
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1219 2732 55 0 (-14)fjt
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1837 2732 55 0 (-12)fjt
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1892 2708 55 0 (/yr)fjt
476 2648 272 1 (Cesium beam)fjt
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1219 2672 55 0 (-13)fjt
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1712 2648 125 2 (3 x 10)fjt
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1837 2672 55 0 (-12)fjt
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1892 2648 55 0 (/yr)fjt
476 2588 362 2 (Rubidium gas cell)fjt
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1219 2612 55 0 (-12)fjt
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1712 2588 125 2 (3 x 10)fjt
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1837 2612 55 0 (-11)fjt
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1892 2588 78 0 (/mo)fjt
476 2528 471 1 (Oven-controlled crystal)fjt
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1219 2552 35 0 (-9)fjt
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1254 2528 234 3 ( 0-50 deg C)fjt
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1837 2552 55 0 (-10)fjt
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476 2468 410 1 (Digital-comp crystal)fjt
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1219 2492 35 0 (-8)fjt
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(C)12 ( )25 (g)22 (e)25 (d)13 ( )25 (0)25 (6)16 (-)25 (0)13 ( )12 ( )0 12 1254 2468 fet
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1837 2492 35 0 (-9)fjt
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476 2408 537 1 (Temp-compensated crystal)fjt
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1219 2432 35 0 (-7)fjt
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1254 2408 234 3 ( 0-60 deg C)fjt
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1837 2432 35 0 (-9)fjt
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476 2348 466 1 (Uncompensated crystal)fjt
1094 2348 152 2 (~1 x 10)fjt
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1246 2372 35 0 (-6)fjt
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1281 2348 206 3 ( per deg C)fjt
1712 2348 184 1 (don't ask)fjt
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804 2251 942 5 (Table 7. Characteristics of Standard Oscillators)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
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300 2242 1840 13 (with those for various types of quartz-crystal oscillators found in electronic equipment. For re)fjt
2140 2242 111 0 (asons)fjt
300 2184 1950 12 (of cost and robustness cesium oscillators are used worldwide for national primary frequency)fjt
300 2126 1878 14 (standards. On the other hand, local clocks used in computing equipment almost always are desig)fjt
2178 2126 72 0 (ned)fjt
300 2067 785 3 (with uncompensated crystal oscillators.)fjt
300 1971 1895 15 (For the three atomic oscillators listed in Table 7 the Drift/Aging column shows the maximum off)fjt
2195 1971 55 0 (set)fjt
300 1913 1819 12 (per day from nominal standard frequency due to systematic mechanical and electrical characte)fjt
2119 1913 131 0 (ristics.)fjt
300 1855 1848 16 (In the case of crystal oscillators this offset is not constant, which results in a gradual chan)fjt
2148 1855 102 1 (ge in)fjt
300 1796 1808 13 (frequency with time, called aging. Even if a crystal oscillator is temperature compensated b)fjt
2108 1796 142 1 (y some)fjt
300 1738 1950 15 (means, it must be periodically compared to a primary standard in order to maintain the highest)fjt
300 1680 1806 14 (accuracy. For all types of oscillators the Stability column shows the maximum variation in fre)fjt
2106 1680 144 0 (quency)fjt
300 1622 1085 8 (per day due to circuit noise and environmental factors.)fjt
300 1516 1886 13 (As the telephone networks of the world are evolving rapidly to digital technology, considerat)fjt
2186 1516 64 0 (ion)fjt
300 1458 1851 14 (should be given to the methods used for frequency synchronization in digital networks. The ind)fjt
2151 1458 100 0 (ustry)fjt
300 1400 1848 14 (has agreed on a classification of clock oscillators as a function of minimum accuracy, mini)fjt
2148 1400 103 0 (mum)fjt
300 1342 1797 13 (stability and other factors [36]. There are three factors which determine the classification: s)fjt
2097 1342 153 0 (tability,)fjt
300 1283 1950 14 (jitter and wander. Stability refers to the systematic variation of frequency with time and is)fjt
300 1225 1950 13 (synonymous with aging, drift, trends, etc. Jitter \(also called timing jitter\) refers to short-term)fjt
300 1167 1950 13 (variations in frequency with components greater than 10 Hz, while wander refers to long-term)fjt
300 1109 1810 13 (variations in frequency with components less than 10 Hz. The classification determines the os)fjt
2110 1109 141 0 (cillator)fjt
300 1050 1889 15 (stratum \(not to be confused with the NTP stratum\), with the more accurate oscillators assigned )fjt
2189 1050 61 0 (the)fjt
300 992 1145 8 (lower strata and less accurate oscillators the higher strata:)fjt
526 863 2024 863 2024 862 526 862 fa
gs eofill gr
525 933 2025 933 2025 932 525 932 fa
gs eofill gr
525 611 2025 611 2025 612 525 612 fa
gs eofill gr
525 933 526 933 526 611 525 611 fa
gs eofill gr
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gs eofill gr
576 872 158 0 (Stratum)fjt
875 872 480 3 (Min Accuracy \(per day\))fjt
1474 872 458 3 (Min Stability \(per day\))fjt
576 802 25 0 (1)fjt
875 802 125 2 (1 x 10)fjt
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1000 826 55 0 (-11)fjt
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1474 802 256 1 (not specified)fjt
576 742 25 0 (2)fjt
875 742 163 2 (1.6 x 10)fjt
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1038 766 35 0 (-8)fjt
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1474 742 125 2 (1 x 10)fjt
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1599 766 55 0 (-10)fjt
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576 682 25 0 (3)fjt
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1038 706 35 0 (-6)fjt
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1474 682 163 2 (3.7 x 10)fjt
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1637 706 35 0 (-7)fjt
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576 622 25 0 (4)fjt
875 622 163 2 (3.2 x 10)fjt
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1038 646 35 0 (-5)fjt
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1474 622 256 1 (not specified)fjt
300 516 1843 12 (The construction, operation and maintenance of stratum-one oscillators is assumed to be cons)fjt
2143 516 108 0 (istent)fjt
300 457 1889 11 (with national standards and often includes cesium oscillators or precision crystal oscillat)fjt
2189 457 61 0 (ors)fjt
300 399 1950 10 (synchronized via LORAN-C to NIST standards. Stratum-two oscillators represent the stability)fjt
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893 2516 90 1 (Osc )fjt
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1021 2120 509 3 (Figure 4. Clock Registers)fjt
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300 2148 1798 15 (exceeding four seconds per day, yet the capability to resolve oscillator drift well below a mill)fjt
2098 2148 152 0 (isecond)fjt
300 2089 1835 10 (per day. These characteristics are appropriate for typical crystal-controlled oscillators w)fjt
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300 2031 1021 5 (without temperature compensation or oven control.)fjt
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300 1943 83 0 (5.3.)fjt
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300 1856 1950 13 (The Fuzzball logical clock is implemented using a 48-bit Clock Register, which increments at)fjt
300 1797 1950 15 (1000-Hz \(at the decimal point\), a 32-bit Clock-Adjust Register, which is used to slew the Clock)fjt
300 1739 1832 13 (Register in response to offset corrections, and a Drift-Compensation Register, which is used t)fjt
2132 1739 118 1 (o trim)fjt
300 1681 1812 14 (the oscillator frequency. In some interface designs such as the DEC KWV11, an additional ha)fjt
2112 1681 138 0 (rdware)fjt
300 1623 1950 14 (Counter Register is used as an auxiliary counter. The configuration and decimal point of these)fjt
300 1564 616 5 (registers are shown in Figure 4.)fjt
916 1564 1334 10 ( The Watchdog Timer and Compliance Register shown in the figure)fjt
300 1506 1950 12 (are used to determine validity, compute frequency corrections and adjust the clock tracking)fjt
300 1448 309 1 (characteristics. )fjt
300 1360 1950 10 (The Clock Register, Clock-Adjust Register and Drift-Compensation Register are implemented in)fjt
300 1302 1950 14 (memory. In typical clock interface designs such as the DEC KWV11, the Counter Register is)fjt
300 1244 1784 13 (implemented as a 16-bit buffered counter driven by a crystal-controlled oscillator at a rate)fjt
2084 1244 166 2 ( of 1000)fjt
300 1186 1925 16 (Hz. A counter overflow is signalled by an interrupt, which results in an increment of the Cloc)fjt
2225 1186 25 0 (k)fjt
300 1127 1950 17 (Register at bit 15. The time of day is determined by reading the Counter Register, which does not)fjt
300 1069 1892 16 (disturb the counting process, and adding its value to that of the Clock Register with decimal poi)fjt
2192 1069 58 0 (nts)fjt
300 1011 1925 15 (aligned. In other interface designs such as the LSI-11 event-line mechanism, each tick of the cloc)fjt
2225 1011 25 0 (k)fjt
300 953 918 8 (is signalled by an interrupt at intervals of 16-)fjt
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1238 953 14 0 (/)fjt
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1252 948 21 0 (3)fjt
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1273 953 977 9 ( ms or 20 ms, depending on interface and mains)fjt
300 894 1911 13 (frequency. When this occurs the appropriate increment in milliseconds, expressed to 32 bits )fjt
2211 894 39 0 (in)fjt
300 836 1950 14 (precision, is added to the Clock Register with decimal points aligned. Monotonicity is insured with)fjt
300 778 669 5 (the parameters shown in Table 8)fjt
969 778 1281 12 (, as long as the increment is at least 2 ms for crystal-stabilized)fjt
300 720 883 6 (clocks or 16 ms for mains-frequency clocks.)fjt
300 632 1824 13 (When the system is initialized all registers, counters and timers are cleared, the leap-indicat)fjt
2124 632 126 1 (or bits)fjt
300 574 435 4 (\(sys.leap\) are set to 11)fjt
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735 569 21 0 (2)fjt
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756 574 1495 9 ( \(unsynchronized\) and the Watchdog Timer begins incrementing at intervals)fjt
300 516 1950 15 (of one second. As each update is received the Watchdog Timer is reset and resumes incrementing)fjt
300 457 1950 15 (from zero. If the value of the Watchdog Timer exceeds NTP.MAXAGE \(one full day\), sys.leap is)fjt
300 399 169 2 (set to 11)fjt
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490 399 13 0 (.)fjt
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451 2828 2098 2828 2098 2827 451 2827 fa
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476 2838 201 0 (Parameter)fjt
1025 2838 119 0 (Name)fjt
1574 2838 144 0 (Crystal)fjt
1849 2838 124 0 (Mains)fjt
476 2768 311 1 (Update Interval)fjt
1025 2768 378 0 (CLOCK.UPDATE)fjt
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476 2708 399 1 (Adjustment Interval)fjt
1025 2708 273 0 (CLOCK.ADJ)fjt
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476 2648 369 1 (Frequency Weight)fjt
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1574 2648 50 0 (10)fjt
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(t)25 (h)25 (g)14 (i)22 (e)47 (W)12 ( )13 ( )22 (e)19 (s)22 (a)25 (h)28 (P)0 13 476 2588 fet
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(t)25 (h)25 (g)13 (i)22 (e)47 (W)13 ( )12 ( )22 (e)22 (c)25 (n)22 (a)14 (i)14 (l)25 (p)39 (m)25 (o)33 (C)0 18 476 2528 fet
1025 2528 351 0 (CLOCK.TRACK)fjt
1574 2528 25 0 (8)fjt
1849 2528 168 1 (not used)fjt
476 2468 461 1 (Compliance Maximum)fjt
1025 2468 323 0 (CLOCK.COMP)fjt
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1849 2468 168 1 (not used)fjt
476 2408 364 1 (Compliance Mask)fjt
1025 2408 326 0 (CLOCK.MASK)fjt
1574 2408 50 0 (37)fjt
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1624 2403 21 0 (8)fjt
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1849 2408 168 1 (not used)fjt
476 2348 397 1 (Maximum Aperture)fjt
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1011 2250 528 3 (Table 8. Clock Parameters)fjt
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300 2841 83 0 (5.4.)fjt
(s)15 (t)30 (n)26 (e)44 (m)15 (t)27 (s)29 (u)13 (j)30 (d)35 (A)12 ( )26 (e)27 (s)27 (a)29 (h)33 (P)12 ( )43 (m)18 (r)30 (o)15 (f)13 (i)30 (n)35 (U)12 ( )0 26 383 2841 fet
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( )25 (n)36 (A)13 ( )12 ( )12 (.)22 (e)14 (t)22 (a)25 (d)25 (p)25 (u)12 ( )14 (t)19 (s)22 (a)14 (l)12 ( )20 (s)13 (t)14 (i)12 ( )17 (f)25 (o)12 ( )25 (y)22 (c)25 (n)22 (e)25 (u)25 (q)22 (e)17 (r)16 (f)12 ( )25 (d)25 (n)22 (a)13 ( )13 (t)22 (e)20 (s)16 (f)17 (f)25 (o)12 ( )22 (e)25 (h)14 (t)12 ( )14 (t)22 (a)12 ( )19 (s)25 (n)25 (u)17 (r)12 ( )25 (k)22 (c)25 (o)14 (l)22 (c)12 ( )14 (l)22 (a)22 (c)13 (i)25 (g)25 (o)14 (l)12 ( )22 (e)25 (h)14 (t)12 ( )13 (,)25 (d)22 (e)14 (t)22 (c)22 (e)16 (r)17 (r)25 (o)22 (c)25 (n)25 (u)12 ( )14 (t)16 (f)22 (e)31 (L)0 93 300 2737 fet
(s)14 (i)12 ( )22 (e)14 (t)22 (a)25 (d)25 (p)25 (u)0 9 2072 2737 fet
300 2679 523 4 (introduced at intervals of 2)fjt
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823 2703 315 0 (CLOCK.UPDATE)fjt
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1138 2679 1113 9 ( seconds as a signed 32-bit integer in milliseconds. When)fjt
300 2621 1950 15 (the magnitude of a correction is less than the maximum aperture CLOCK.MAX, bits 16-31 of the)fjt
300 2563 1829 15 (update replace bits 0-15 of the Clock-Adjust Register. In order to minimize the effects of trun)fjt
2129 2563 122 0 (cation)fjt
300 2504 1731 19 (and roundoff errors, bits 16-31 are set to zeros if the sign of the update is positive and ones )fjt
2031 2504 219 1 (if negative.)fjt
300 2446 1928 16 (In addition, the update is also divided by a weighting factor \(described later\) and added to th)fjt
2228 2446 22 0 (e)fjt
300 2388 1146 6 (Drift-Compensation Register. At adjustment intervals of 2)fjt
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1446 2412 227 0 (CLOCK.ADJ)fjt
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1673 2388 561 4 ( seconds a correction consist)fjt
2234 2388 17 0 (-)fjt
300 2330 1950 15 (ing of two components is computed. The first \(phase\) component consists of the value of the)fjt
300 2271 1950 11 (Clock-Adjust Register shifted right CLOCK.PHASE bits, which is then subtracted from the)fjt
300 2213 1934 12 (Clock-Adjust Register. The second \(frequency\) component consists of the value of the Drift-Com)fjt
2234 2213 17 0 (-)fjt
300 2155 1950 12 (pensation Register shifted right by the quantity CLOCK.FREQ - CLOCK.UPDATE. The sum of)fjt
300 2097 1938 15 (the phase and frequency components is the correction, which is then added to the Clock Register)fjt
2238 2097 13 0 (.)fjt
300 2038 1365 10 (Operation continues in this way until a new correction is introduced.)fjt
300 1919 1900 16 (For the ultimate stability of about a millisecond per day in the absence of outside updates, it)fjt
2200 1919 50 1 ( is)fjt
300 1861 1898 15 (necessary to reduce the influence of the frequency component once the clock has been running w)fjt
2198 1861 53 0 (ith)fjt
300 1803 1950 17 (low offsets for some time. This is done only in the case of crystal oscillators and using the)fjt
300 1744 1950 13 (Compliance Register, which contains an exponential average of all prior updates. The average is)fjt
300 1686 1681 13 (computed by first shifting the update left eight bits for efficient scaling, then subtractin)fjt
1981 1686 269 2 (g the contents)fjt
300 1628 1950 13 (of the Compliance Register, then shifting the result right CLOCK.TRACK bits and finally adding)fjt
300 1570 752 5 (the result to the Compliance Register.)fjt
300 1450 1912 15 (When the update is added to the Drift Compensation Register, the value in the Compliance Regist)fjt
2212 1450 39 0 (er)fjt
300 1392 1889 18 (is copied to a temporary and the low-order bit set to one. Both the update and temporary are shif)fjt
2189 1392 61 0 (ted)fjt
300 1334 1950 13 (left together until the bitwise AND of the temporary and the mask ~CLOCK.MASK become)fjt
300 1276 1931 15 (nonzero. The parameters in Table 7 have been selected so that, under good conditions with update)fjt
2231 1276 19 0 (s)fjt
300 1217 1807 18 (in the order of a few milliseconds, a precision of a millisecond per day \(about .01 ppm or 10)fjt
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2107 1241 35 0 (-8)fjt
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2141 1217 109 1 (\), can)fjt
300 1159 1950 11 (be achieved. In the case of mains-frequency clocks the Compliance Register, CLOCK.TRACK,)fjt
300 1101 1203 6 (CLOCK.COMP and CLOCK.MASK variables are not used.)fjt
300 982 1950 13 (When mains-frequency oscillators must be used, the loop parameters must be adapted for the)fjt
300 923 1950 13 (relatively high jitter and wander characteristics of the regional power grid, in which diurnal)fjt
300 865 1950 13 (peak-to-peak phase excursions can exceed four seconds. Simulation of a loop with the parameters)fjt
300 807 1837 17 (of Figure 6 and the clock filter described in Section 4 results in a transient response similar )fjt
2137 807 113 1 (to the)fjt
300 749 1950 11 (crystal-stabilized case, but with somewhat smaller time constants. When presented with actual)fjt
300 690 1936 15 (phase-offset data from the U.S. Eastern, U.S. Western and West German power grids and for typica)fjt
2236 690 14 0 (l)fjt
300 632 1864 18 (summer days when the jitter and wander are the largest, the residual errors are in the order of a)fjt
2164 632 86 1 ( few)fjt
300 574 1950 14 (tens of milliseconds, but seldom more than 100 ms. With mains-frequency oscillators it is not)fjt
300 516 1822 14 (possible to increase the polling interval above a minute or so without significant increase i)fjt
2122 516 128 1 (n loop)fjt
300 457 1853 13 (error or degradation of transient response, so the polling interval peer.hpoll is always clamp)fjt
2153 457 97 1 (ed at)fjt
300 399 341 0 (NTP.MINPOLL.)fjt
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300 2841 1865 15 (Care is required in the implementation to insure monotonicity of the Clock Register and to pres)fjt
2165 2841 86 0 (erve)fjt
300 2782 1950 13 (the highest precision while minimizing the propagation of roundoff errors. Since all of the)fjt
300 2724 1834 11 (multiply/divide operations can be approximated by bitwise-shift operations, it is not necess)fjt
2134 2724 117 1 (ary to)fjt
300 2666 1831 12 (implement a full multiply/divide capability in hardware or software. In the various implement)fjt
2131 2666 119 0 (ations)fjt
300 2608 1950 15 (of NTP for many Unix-based systems it has been the common experience that the single most)fjt
300 2549 1818 13 (important factor affecting local-clock stability is the matching of the phase and frequency )fjt
2118 2549 116 0 (coeffi)fjt
2234 2549 17 0 (-)fjt
300 2491 1843 13 (cients to the particular kernel implementation. It is vital that these coefficients be engin)fjt
2143 2491 108 0 (eered)fjt
300 2433 1859 15 (according to the model values, for otherwise the phase-lock loop can fail to track normal oscil)fjt
2159 2433 91 0 (lator)fjt
300 2375 827 5 (variations and can even become unstable.)fjt
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300 2276 83 0 (5.5.)fjt
(s)16 (t)29 (n)27 (e)43 (m)16 (t)26 (s)30 (u)13 (j)29 (d)35 (A)12 ( )27 (e)27 (s)26 (a)30 (h)32 (P)12 ( )44 (m)18 (r)29 (o)16 (f)13 (i)29 (n)30 (u)29 (n)30 (o)35 (N)12 ( )0 29 383 2276 fet
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300 2184 1950 12 (When the magnitude of a correction exceeds the maximum aperture CLOCK.MAX, the possibility)fjt
300 2125 1806 17 (exists that the clock is so far out of synchronization with the reference source that the best a)fjt
2106 2125 144 1 (ction is)fjt
300 2067 1850 13 (an immediate and wholesale replacement of Clock Register contents, rather than a graduated sle)fjt
2150 2067 100 0 (wing)fjt
300 2009 1879 16 (as described above. If this happens, the Clock-Adjust Register is set to zero, but the other regis)fjt
2179 2009 72 0 (ters)fjt
300 1951 1813 13 (remain undisturbed. In addition, as described previously, the clear procedure is called to pu)fjt
2113 1951 137 1 (rge the)fjt
300 1892 1772 15 (clock filters and estimation variables for all peers. In practice, the necessity to do this is)fjt
2072 1892 178 2 ( rare and)fjt
300 1834 1855 15 (usually occurs when the local host or reference source is rebooted, for example. This is fortu)fjt
2155 1834 95 0 (nate,)fjt
300 1776 1950 16 (since step changes in the clock can result in the clock apparently running backward, as well as)fjt
300 1718 1624 9 (incorrect delay and offset measurements of the synchronization mechanism itself.)fjt
300 1620 1950 10 (Considerable experience with the Internet environment suggests the values of CLOCK.MAX)fjt
300 1562 1848 15 (tabulated in Table 7 as appropriate. In practice, these values are exceeded with a single time-s)fjt
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300 1504 1878 14 (source only under conditions of the most extreme congestion or when multiple failures of node)fjt
2178 1504 72 1 (s or)fjt
300 1445 1950 15 (links have occurred. The most common case when the maximum is exceeded is when the time-server)fjt
300 1387 1950 17 (source is changed and the time indicated by the new and old sources exceeds the maximum due to)fjt
300 1329 1889 13 (systematic errors in the primary reference source or large differences in the synchronizing p)fjt
2189 1329 61 0 (ath)fjt
300 1271 1950 11 (delays. It is recommended that implementations include provisions to tailor CLOCK.MAX for)fjt
(e)25 (h)14 (t)25 ( )25 (g)26 (n)14 (i)14 (t)22 (a)14 (l)25 (o)14 (i)25 (v)25 ( )14 (t)26 (u)25 (o)25 (h)14 (t)14 (i)36 (w)25 ( )26 (d)22 (e)19 (s)23 (a)22 (e)17 (r)22 (c)25 (n)14 (i)25 ( )22 (e)26 (b)25 ( )25 (n)22 (a)22 (c)25 ( )37 (X)36 (A)44 (M)13 (.)36 (K)34 (C)36 (O)31 (L)33 (C)25 ( )14 (t)23 (a)25 (h)14 (t)25 ( )14 (t)25 (n)26 (u)25 (o)40 (m)22 (a)25 ( )23 (e)25 (h)31 (T)25 ( )13 (.)20 (s)25 (n)26 (o)14 (i)14 (t)23 (a)25 (u)15 (t)14 (i)20 (s)25 ( )22 (c)14 (i)17 (f)15 (i)22 (c)23 (e)25 (p)20 (s)0 85 300 1212 fet
300 1154 1931 14 (monotonicity requirement depends on the Clock Register increment. For an increment of 10 ms, a)fjt
2231 1154 19 0 (s)fjt
300 1096 1818 17 (used in many workstations, the value shown in Table 7 can be increased by a factor of five.)fjt
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(e)43 (m)13 (i)28 (T)13 ( )30 (d)29 (n)27 (a)13 ( )26 (e)16 (t)27 (a)35 (D)12 ( )30 (g)29 (n)13 (i)30 (n)12 (i)27 (a)16 (t)29 (n)13 (i)27 (a)40 (M)13 ( )0 26 383 997 fet
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300 905 1950 14 (Conversion from NTP format to the common date and time formats used by application programs)fjt
300 846 1801 15 (is simplified if the internal local-clock format uses separate date and time registers. The time )fjt
2101 846 149 0 (register)fjt
300 788 1909 18 (is designed to roll over at 24 hours, give or take a leap second as determined by the leap-indicat)fjt
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300 730 1878 12 (bits, with its overflows \(underflows\) incrementing \(decrementing\) the date register. The date )fjt
2178 730 72 0 (and)fjt
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2238 672 13 0 (,)fjt
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300 516 1788 18 (On the day prior to the insertion of a leap second the leap-indicator bits are set at the primary)fjt
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300 457 1950 16 (presumably by manual means. Subsequently, these bits show up at the local host and are passed to)fjt
300 399 1928 16 (the local-clock procedure. This causes the modulus of the time register, which is the length of th)fjt
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300 2782 1892 18 (insertion the bits are turned off at the primary servers. While it is possible to turn the bits )fjt
2192 2782 58 0 (off)fjt
300 2724 1828 14 (automatically, the procedure suggested here insures that all clocks have rolled over and will )fjt
2128 2724 123 1 (not be)fjt
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2017 2666 233 2 ( of rollover.)fjt
300 2577 1950 15 (Lack of a comprehensive mechanism to administer the leap bits in the primary time servers is)fjt
300 2519 1950 10 (presently an awkward problem better suited to a comprehensive network-management mechanism)fjt
300 2460 1883 15 (yet to be developed. As a practical matter and unless specific provisions have been made otherw)fjt
2183 2460 68 0 (ise,)fjt
300 2402 1950 12 (currently manufactured radio clocks have no provisions for leap seconds, automatic, manual or)fjt
300 2344 1773 13 (otherwise. Therefore, the only possible solution is to disable the radio clock immediately f)fjt
2073 2344 177 0 (ollowing)fjt
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300 2046 1950 13 (Many people contributed to the contents of this document, which was thoroughly debated by)fjt
300 1988 1950 11 (electronic mail and debugged using prototype implementations written by Louis Mamakos and)fjt
300 1930 1934 16 (Michael Petry of the University of Maryland for the Unix 4.3bsd operating system and by the autho)fjt
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300 2839 63 0 (42.)fjt
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375 2781 1600 9 (Network Working Group Report RFC-1059, University of Delaware, July 1988.)fjt
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1042 2338 417 3 ( \(April 1989\), 35-45.)fjt
greset -300 3599 2850 3599 2850 -301 -300 -301 np mto lto lto lto clip np
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300 2841 42 0 (8.)fjt
(2)12 ( )30 (n)29 (o)13 (i)27 (s)18 (r)27 (e)29 (V)13 ( )15 (-)12 ( )16 (t)27 (a)43 (m)18 (r)30 (o)29 (F)12 ( )27 (a)16 (t)26 (a)35 (D)13 ( )31 (P)30 (T)35 (N)12 ( )13 (.)35 (A)12 ( )27 (x)13 (i)29 (d)30 (n)26 (e)30 (p)29 (p)35 (A)13 ( )0 40 350 2841 fet
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greset -300 3599 2850 3599 2850 -301 -300 -301 np mto lto lto lto clip np
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800 1413 801 1413 801 1338 800 1338 fa
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gs eofill gr
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875 1413 876 1413 876 1338 875 1338 fa
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gs eofill gr
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gs eofill gr
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gs eofill gr
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gs eofill gr
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877 1354 72 0 (VN)fjt
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gs eofill gr
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gs eofill gr
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1500 1413 1501 1413 1501 1338 1500 1338 fa
colmap 1 [0 0 0 ] put
1 sci
gs eofill gr
1500 1413 1725 1413 1725 1412 1500 1412 fa
gs eofill gr
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gs eofill gr
1725 1413 1726 1413 1726 1338 1725 1338 fa
gs eofill gr
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colmap 1 [0 0 0 ] put
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gs eofill gr
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gs eofill gr
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gs eofill gr
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/tface 8 def
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colmap 1 [0 0 0 ] put
1 sci
gs eofill gr
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gs eofill gr
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gs eofill gr
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gs eofill gr
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colmap 1 [0 0 0 ] put
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gs eofill gr
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800 738 801 738 801 613 800 613 fa
colmap 1 [0 0 0 ] put
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gs eofill gr
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gs eofill gr
greset 730 813 1791 813 1791 539 730 539 np mto lto lto lto clip np
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800 1113 801 1113 801 988 800 988 fa
colmap 1 [0 0 0 ] put
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gs eofill gr
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gs eofill gr
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800 988 801 988 801 863 800 863 fa
colmap 1 [0 0 0 ] put
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gs eofill gr
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gs eofill gr
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gs eofill gr
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800 863 801 863 801 738 800 738 fa
colmap 1 [0 0 0 ] put
1 sci
gs eofill gr
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gs eofill gr
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gs eofill gr
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/tface 8 def
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1016 786 492 2 (Receive Timestamp \(64\))fjt
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colmap 1 [0 0 0 ] put
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gs eofill gr
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gs eofill gr
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gs eofill gr
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/tface 8 def
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800 1188 801 1188 801 1113 800 1113 fa
colmap 1 [0 0 0 ] put
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gs eofill gr
800 1188 1725 1188 1725 1187 800 1187 fa
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800 1113 1725 1113 1725 1112 800 1112 fa
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1725 1188 1726 1188 1726 1113 1725 1113 fa
gs eofill gr
greset 730 1263 1791 1263 1791 1039 730 1039 np mto lto lto lto clip np
/tface 8 def
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1015 1129 494 2 (Reference Identifier \(32\))fjt
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greset 730 1533 1791 1533 1791 442 730 442 np mto lto lto lto clip np
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greset 730 1533 1791 1533 1791 442 730 442 np mto lto lto lto clip np
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colmap 1 [0 0 0 ] put
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greset 730 1533 1791 1533 1791 442 730 442 np mto lto lto lto clip np
800 613 801 613 801 488 800 488 fa
colmap 1 [0 0 0 ] put
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gs eofill gr
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gs eofill gr
800 488 1725 488 1725 487 800 487 fa
gs eofill gr
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gs eofill gr
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/tface 8 def
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973 536 578 2 (Authenticator \(optional\) \(96\))fjt
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961 400 629 4 (Figure 5. NTP Message Header)fjt
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colmap 1 [0 0 0 ] put
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
greset -300 3599 2850 3599 2850 -301 -300 -301 np mto lto lto lto clip np
greset -75 474 2624 474 2624 -75 -75 -75 np mto lto lto lto clip np
/tface 4 def
colmap 1 [0 0 0 ] put
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300 279 100 0 (Mills)fjt
2065 279 186 1 (Page 45)fjt
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8 encfont
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533 2839 25 0 (7)fjt
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300 2742 1792 15 (Stratum: This is a eight-bit integer indicating the stratum level of the local clock, with values)fjt
2092 2742 158 1 ( defined)fjt
375 2682 217 1 (as follows:)fjt
533 2583 25 0 (0)fjt
700 2583 229 0 (unspecified)fjt
533 2523 25 0 (1)fjt
700 2523 720 4 (primary reference \(e.g., radio clock\))fjt
533 2463 117 0 (2-255)fjt
700 2463 612 3 (secondary reference \(via NTP\))fjt
375 2366 1608 13 (The values that can appear in this field range from zero to NTP.INFIN inclusive.)fjt
300 2260 1732 12 (Poll Interval: This is an eight-bit signed integer indicating the maximum interval between)fjt
2032 2260 218 1 ( successive)fjt
375 2200 1875 17 (messages, in seconds to the nearest power of two. The values that can appear in this field range)fjt
375 2140 1054 4 (from NTP.MINPOLL to NTP.MAXPOLL inclusive.)fjt
300 2034 1834 15 (Precision: This is an eight-bit signed integer indicating the precision of the local clock, in se)fjt
2134 2034 116 0 (conds)fjt
375 1974 556 5 (to the nearest power of two.)fjt
300 1867 1835 11 (Synchronizing Distance: This is a 32-bit fixed-point number indicating the estimated roundtrip)fjt
2135 1867 116 1 ( delay)fjt
375 1807 1820 14 (to the primary synchronizing source, in seconds with fraction point between bits 15 and 16.)fjt
300 1701 1851 11 (Synchronizing Dispersion: This is a 32-bit fixed-point number indicating the estimated dispe)fjt
2151 1701 100 0 (rsion)fjt
375 1641 1820 14 (to the primary synchronizing source, in seconds with fraction point between bits 15 and 16.)fjt
300 1534 1807 12 (Reference Clock Identifier: This is a 32-bit code identifying the particular reference clock)fjt
2107 1534 143 2 (. In the)fjt
375 1474 1772 14 (case of stratum 0 \(unspecified\) or stratum 1 \(primary reference\), this is a four-octet, left-just)fjt
2147 1474 104 0 (ified,)fjt
375 1414 1875 12 (zero-padded ASCII string. While not ennumerated as part of the NTP specification, the)fjt
375 1354 846 4 (following are suggested ASCII identifiers:)fjt
651 1225 1900 1225 1900 1224 651 1224 fa
gs eofill gr
650 1296 1901 1296 1901 1295 650 1295 fa
gs eofill gr
650 554 1901 554 1901 555 650 555 fa
gs eofill gr
650 1296 651 1296 651 554 650 554 fa
gs eofill gr
1901 1296 1900 1296 1900 554 1901 554 fa
gs eofill gr
700 1235 158 0 (Stratum)fjt
978 1235 105 0 (Code)fjt
1255 1235 177 0 (Meaning)fjt
700 1165 25 0 (0)fjt
978 1165 105 0 (DCN)fjt
1255 1165 440 2 (DCN routing protocol)fjt
700 1105 25 0 (0)fjt
978 1105 111 0 (NIST)fjt
1255 1105 410 2 (NIST public modem)fjt
700 1045 25 0 (0)fjt
978 1045 86 0 (TSP)fjt
1255 1045 365 2 (TSP time protocol)fjt
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375 457 1802 15 (In the case of type 2 and greater \(secondary reference\) this is the four-octet Internet addres)fjt
2177 457 73 1 (s of)fjt
375 399 366 2 (the reference host.)fjt
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300 2839 1911 17 (Reference Timestamp: This is the local time at which the local clock was last set or corrected, )fjt
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375 2779 496 2 (64-bit timestamp format.)fjt
300 2689 1950 16 (Originate Timestamp: This is the local time at which the request departed the client host for the)fjt
375 2629 808 5 (service host, in 64-bit timestamp format.)fjt
300 2539 1856 17 (Receive Timestamp: This is the local time at which the request arrived at the service host, in 6)fjt
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375 2479 365 1 (timestamp format.)fjt
300 2389 1862 17 (Transmit Timestamp: This is the local time at which the reply departed the service host for the c)fjt
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375 2329 655 4 (host, in 64-bit timestamp format.)fjt
300 2239 1931 10 (Authenticator \(optional\): When the NTP authentication mechanism is implemented, this contain)fjt
2231 2239 19 0 (s)fjt
375 2179 1061 6 (the authenticator information defined in Appendix C.)fjt
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300 2751 1870 10 (In a comprehensive network-management environment, facilities are presumed available to perf)fjt
2170 2751 80 0 (orm)fjt
300 2692 1834 14 (routine NTP control and monitoring functions, such as setting the leap-indicator bits at the pr)fjt
2134 2692 116 0 (imary)fjt
300 2634 1913 10 (servers, adjusting the various system parameters and monitoring regular operations. Ordinaril)fjt
2213 2634 38 0 (y,)fjt
300 2576 1950 12 (these functions can be implemented using a network-management protocol such as SNMP and)fjt
300 2518 1950 14 (suitable extensions to the MIB database. However, in those cases where such facilities are not)fjt
300 2459 1860 12 (available, these functions can be implemented using special NTP control messages described he)fjt
2160 2459 90 0 (rein.)fjt
300 2401 1950 14 (These messages are intended for use only in systems where no other management facilities are)fjt
300 2343 1862 12 (available or appropriate, such as in dedicated-function bus peripherals. Support for these mess)fjt
2162 2343 88 0 (ages)fjt
300 2285 1103 9 (is not required in order to conform to this specification.)fjt
300 2192 1950 19 (The NTP Control Message has the value 6 specified in the mode field of the first octet of the NTP)fjt
300 2134 1950 17 (header and is formatted as shown below. The format of the data field is specific to each command)fjt
300 2075 1950 16 (or response; however, in most cases the format is designed to be constructed and viewed by humans)fjt
300 2017 1889 14 (and so is coded in free-form ASCII. This facilitates the specification and implementation of sim)fjt
2189 2017 61 0 (ple)fjt
300 1959 1950 12 (management tools in the absence of fully evolved network-management facilities. As in ordinary)fjt
300 1901 1892 15 (NTP messages, the authenticator field follows the data field. If the authenticator is used the d)fjt
2192 1901 58 0 (ata)fjt
300 1842 1914 17 (field is zero-padded to a 32-bit boundary, but the padding bits are not considered part of the da)fjt
2214 1842 36 0 (ta)fjt
300 1784 873 8 (field and are not included in the field count.)fjt
300 1691 1931 14 (IP hosts are not required to reassemble datagrams larger than 576 octets; however, some command)fjt
2231 1691 19 0 (s)fjt
300 1633 1950 15 (or responses may involve more data than will fit into a single datagram. Accordingly, a simple)fjt
300 1575 1898 15 (reassembly feature is included in which each octet of the message data is numbered starting w)fjt
2198 1575 53 0 (ith)fjt
300 1517 1791 17 (zero. As each fragment is transmitted the number of its first octet is inserted in the offset f)fjt
2091 1517 159 1 (ield and)fjt
300 1458 1917 18 (the number of octets is inserted in the count field. The more-data \(M\) bit is set in all fragmen)fjt
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300 1400 297 2 (except the last.)fjt
300 1307 1925 12 (Most control functions involve sending a command and receiving a response, perhaps involvin)fjt
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300 1249 1876 14 (several fragments. The sender chooses a distinct, nonzero sequence number and sets the status f)fjt
2176 1249 75 0 (ield)fjt
300 1191 1785 15 (and R and E bits to zero. The responder interprets the opcode and additional information in)fjt
2085 1191 165 2 ( the data)fjt
300 1132 1865 19 (field, updates the status field, sets the R bit to one and returns the three 32-bit words of the he)fjt
2165 1132 86 0 (ader)fjt
300 1074 1806 15 (along with additional information in the data field. In case of invalid message format or c)fjt
2106 1074 144 0 (ontents)fjt
300 1016 1821 19 (the responder inserts a code in the status field, sets the R and E bits to one and, optionally, )fjt
2121 1016 130 0 (inserts)fjt
300 958 756 6 (a diagnostic message in the data field.)fjt
300 865 1898 13 (Some commands read or write system variables and peer variables for an association identified)fjt
2198 865 52 1 ( in)fjt
300 807 1925 15 (the command. Others read or write variables associated with a radio clock or other device directl)fjt
2225 807 25 0 (y)fjt
300 749 1889 13 (connected to a source of primary synchronization information. To identify which type of varia)fjt
2189 749 61 0 (ble)fjt
300 690 1699 13 (and association a 16-bit association identifier is used. System variables are indicated by )fjt
1999 690 251 1 (the identifier)fjt
300 632 1749 15 (zero. As each association is mobilized a unique, nonzero identifier is created for it. These )fjt
2049 632 201 0 (identifiers)fjt
300 574 1950 18 (are used in a cyclic fashion, so that the chance of using an old identifier which matches a newly)fjt
300 516 1878 14 (created association is remote. A management entity can request a list of current identifiers )fjt
2178 516 72 0 (and)fjt
300 457 1848 16 (subsequently use them to read and write variables for each association. An attempt to use an ex)fjt
2148 457 102 0 (pired)fjt
300 399 1803 13 (identifier results in an exception response, following which the list can be requested again.)fjt
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300 2050 1892 13 (Some exception events, such as when a peer becomes reachable or unreachable, occur spontaneou)fjt
2192 2050 58 0 (sly)fjt
300 1992 1889 15 (and are not necessarily associated with a command. An implementation may elect to save the ev)fjt
2189 1992 61 0 (ent)fjt
300 1933 1887 16 (information for later retrieval or to send an asynchronous response \(called a trap\) or both. In c)fjt
2187 1933 63 0 (ase)fjt
300 1875 1950 17 (of a trap the IP address and port number is determined by a previous command and the sequence)fjt
300 1817 1886 14 (field is set as described below. Current status and summary information for the latest except)fjt
2186 1817 64 0 (ion)fjt
300 1759 1808 15 (event is returned in all normal responses. Bits in the status field indicate whether an except)fjt
2108 1759 143 1 (ion has)fjt
300 1700 1668 12 (occurred since the last response and whether more than one exception has occurred.)fjt
300 1610 1855 13 (Commands need not necessarily be sent by an NTP peer, so ordinary access-control procedures)fjt
2155 1610 95 1 ( may)fjt
300 1552 1950 11 (not apply; however, the optional mask/match mechanism suggested elsewhere in this document)fjt
300 1493 1892 17 (provides the capability to control access by mode number, so this could be used to limit access )fjt
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300 1435 1087 8 (control messages \(mode 6\) to selected address ranges. )fjt
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300 1255 1950 14 (The format of the NTP Control Message header, which immediately follows the UDP header, is)fjt
300 1196 367 3 (shown in Figure 6.)fjt
667 1196 1583 14 ( Following is a description of its fields. Bit positions marked as zero are reserved)fjt
300 1138 828 6 (and should always be transmitted as zero.)fjt
300 1046 1840 13 (Version Number \(VN\): This is a three-bit integer indicating the NTP version number, currentl)fjt
2140 1046 110 1 (y two)fjt
375 986 71 0 (\(2\).)fjt
300 892 1856 17 (Mode: This is a three-bit integer indicating the mode. It must have the value 6, indicating an )fjt
2156 892 94 0 (NTP)fjt
375 832 334 1 (control message.)fjt
300 739 1266 10 (Response Bit \(R\): Set to zero for commands, one for responses.)fjt
300 646 1379 12 (Error Bit \(E\): Set to zero for normal response, one for error response.)fjt
300 552 1230 12 (More Bit \(M\): Set to zero for last fragment, one for all others.)fjt
300 459 1898 13 (Operation Code \(Op\): This is a five-bit integer specifying the command function. Values curren)fjt
2198 459 53 0 (tly)fjt
375 399 602 3 (defined include the following:)fjt
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gs eofill gr
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colmap 1 [0 0 0 ] put
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colmap 1 [0 0 0 ] put
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colmap 1 [0 0 0 ] put
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gs eofill gr
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gs eofill gr
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colmap 1 [0 0 0 ] put
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gs eofill gr
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973 2276 578 2 (Authenticator \(optional\) \(96\))fjt
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gs eofill gr
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967 1908 616 4 (Figure 7. Status Word Formats)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
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300 2841 83 0 (9.2.)fjt
(s)29 (d)18 (r)30 (o)45 (W)13 ( )27 (s)29 (u)16 (t)26 (a)16 (t)32 (S)13 ( )0 13 383 2841 fet
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300 2742 1853 14 (Status words indicate the present status of the system, associations and clock. They are design)fjt
2153 2742 97 1 (ed to)fjt
300 2684 1950 14 (be interpreted by network-monitoring programs and are in one of four 16-bit formats shown in)fjt
300 2626 755 6 (Figure 7 and described in this section.)fjt
1055 2626 1195 9 ( System and peer status words are associated with responses)fjt
300 2567 1920 14 (for all commands except the read clock variables, write clock variables and set trap address/po)fjt
2220 2567 30 0 (rt)fjt
300 2509 1950 12 (commands. The association identifier zero specifies the system status word, while a nonzero)fjt
300 2451 1831 13 (identifier specifies a particular peer association. The status word returned in response to read)fjt
2131 2451 119 1 ( clock)fjt
300 2393 1861 14 (variables and write clock variables commands indicates the state of the clock hardware and deco)fjt
2161 2393 89 0 (ding)fjt
300 2334 1896 15 (software. A special error status word is used to report malformed command fields or invalid valu)fjt
2196 2334 54 0 (es.)fjt
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300 2224 125 0 (9.2.1.)fjt
(d)18 (r)29 (o)46 (W)12 ( )27 (s)30 (u)15 (t)27 (a)15 (t)33 (S)12 ( )44 (m)26 (e)16 (t)27 (s)26 (y)33 (S)12 ( )0 19 425 2224 fet
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300 2113 1870 18 (The system status word appears in the status field of the response to a read status or read varia)fjt
2170 2113 80 0 (bles)fjt
300 2054 1865 15 (command with a zero association identifier. The format of the system status word is as follow)fjt
2165 2054 33 0 (s:)fjt
300 1943 1934 16 (Leap Indicator \(LI\): This is a two-bit code warning of an impending leap second to be in)fjt
2234 1943 17 0 (-)fjt
375 1883 1856 17 (serted/deleted in the last minute of the current day, with bit 0 and bit 1, respectively, coded a)fjt
2231 1883 19 0 (s)fjt
375 1823 163 0 (follows:)fjt
533 1721 50 0 (00)fjt
700 1721 226 1 (no warning)fjt
533 1661 50 0 (01)fjt
700 1661 531 4 (last minute has 61 seconds)fjt
533 1601 50 0 (10)fjt
700 1601 547 4 (last minute has 59 seconds\))fjt
533 1541 50 0 (11)fjt
700 1541 821 4 (alarm condition \(clock not synchronized\))fjt
300 1441 1884 13 (Clock Source: This is a six-bit integer indicating the current synchronization source, with val)fjt
2184 1441 66 0 (ues)fjt
375 1381 348 2 (coded as follows:)fjt
533 1280 25 0 (0)fjt
700 1280 482 2 (unspecified or unknown)fjt
533 1220 25 0 (1)fjt
700 1220 634 5 (VLF \(band 4\) radio \(e.g., GBR\))fjt
533 1160 25 0 (2)fjt
700 1160 659 5 (LF \(band 5\) radio \(e.g., WWVB\))fjt
533 1100 25 0 (3)fjt
700 1100 936 7 (HF \(band 7\) radio \(e.g., CHU, MSF, WWV/H\))fjt
533 1040 25 0 (4)fjt
700 1040 836 6 (UHF \(band 9\) satellite \(e.g., GOES, GPS\))fjt
533 980 25 0 (5)fjt
700 980 529 4 (local net \(e.g., DCN, TSP\))fjt
533 920 25 0 (6)fjt
700 920 208 0 (UDP/NTP)fjt
533 860 25 0 (7)fjt
700 860 235 0 (UDP/TIME)fjt
533 800 25 0 (8)fjt
700 800 467 0 (eyeball-and-wristwatch)fjt
533 740 25 0 (9)fjt
700 740 609 3 (telephone modem \(e.g., NIST\))fjt
533 680 117 0 (10-63)fjt
700 680 168 0 (reserved)fjt
300 579 1845 14 (System Event Counter: This is a four-bit integer indicating the number of system exception e)fjt
2145 579 105 0 (vents)fjt
375 519 1875 17 (occuring since the last time the system status word was returned in a response or included in a)fjt
375 459 1875 16 (trap message. The counter is cleared when returned in the status field of a response and freezes)fjt
375 399 578 5 (when it reaches the value 15.)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
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300 2839 1862 14 (System Event Code: This is a four-bit integer identifying the latest system exception event, )fjt
2162 2839 89 0 (with)fjt
375 2779 1249 8 (new values overwriting previous values, and coded as follows:)fjt
533 2684 25 0 (0)fjt
700 2684 229 0 (unspecified)fjt
533 2624 25 0 (1)fjt
700 2624 274 1 (system restart)fjt
533 2564 25 0 (2)fjt
700 2564 493 3 (system or hardware fault)fjt
533 2504 25 0 (3)fjt
700 2504 1225 8 (system new status word \(leap bits or synchronization change\))fjt
533 2444 25 0 (4)fjt
700 2444 1194 8 (system new clock source or stratum \(sys.peer or sys.stratum)fjt
700 2384 158 0 (change\))fjt
533 2324 25 0 (5)fjt
700 2324 1217 6 (system clock reset \(offset correction exceeds CLOCK.MAX\))fjt
533 2264 25 0 (6)fjt
700 2264 922 7 (system invalid time or date \(see Section 3.4.5\))fjt
533 2204 25 0 (7)fjt
700 2204 1084 7 (system clock exception \(see system clock status word\))fjt
533 2144 92 0 (8-15)fjt
700 2144 168 0 (reserved)fjt
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300 2050 125 0 (9.2.2.)fjt
(d)18 (r)30 (o)45 (W)13 ( )26 (s)30 (u)15 (t)27 (a)16 (t)32 (S)13 ( )18 (r)27 (e)26 (e)33 (P)12 ( )0 17 425 2050 fet
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300 1952 1822 19 (A peer status word is returned in the status field of a response to a read status, read variables o)fjt
2122 1952 128 1 (r write)fjt
300 1894 1862 14 (variables command and appears also in the list of association identifers and status words retu)fjt
2162 1894 89 0 (rned)fjt
300 1835 1877 17 (by a read status command with a zero association identifier. The format of a peer status word i)fjt
2177 1835 73 1 (s as)fjt
300 1777 163 0 (follows:)fjt
300 1678 1950 16 (Peer Status: This is a six-bit code indicating the status of the peer determined by the packet)fjt
375 1618 810 5 (procedure, with bits assigned as follows:)fjt
533 1523 25 0 (0)fjt
700 1523 487 1 (configured \(peer.config\))fjt
533 1463 25 0 (1)fjt
700 1463 808 2 (authentication enabled \(peer.authenable\))fjt
533 1403 25 0 (2)fjt
700 1403 608 1 (authentication \(peer.authentic\))fjt
533 1343 25 0 (3)fjt
700 1343 591 3 (reachability okay \(peer.reach )fjt
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1291 1343 27 0 (\271)fjt
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1318 1343 54 1 ( 0\))fjt
533 1283 25 0 (4)fjt
700 1283 615 3 (sanity okay \(packet procedure\))fjt
533 1223 25 0 (5)fjt
700 1223 1159 5 (dispersion okay \(peer.dispersion < PEER.THRESHOLD\) )fjt
300 1129 1815 14 (Peer Selection \(Select\): This is a two-bit integer indicating the status of the peer determined)fjt
2115 1129 135 2 ( by the)fjt
375 1069 1126 6 (clock-selection procedure, with values coded as follows:)fjt
533 973 25 0 (0)fjt
700 973 157 0 (rejected)fjt
533 913 25 0 (1)fjt
700 913 1208 8 (selection candidate \(survivor of the pruned and truncated list)fjt
700 853 580 2 (sorted by stratum/dispersion\))fjt
533 793 25 0 (2)fjt
700 793 1233 8 (synchronization candidate \(survivor of the list sorted by delay)fjt
700 733 420 2 (less outlyer discards\))fjt
533 673 25 0 (3)fjt
700 673 403 2 (current clock source)fjt
300 579 1876 15 (Peer Event Counter: This is a four-bit integer indicating the number of peer exception events )fjt
2176 579 75 0 (that)fjt
375 519 1875 18 (occured since the last time the peer status word was returned in a response or included in a trap)fjt
375 459 1875 16 (message. The counter is cleared when returned in the status field of a response and freezes when)fjt
375 399 457 4 (it reaches the value 15.)fjt
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300 2839 1892 15 (Peer Event Code: This is a four-bit integer identifying the latest peer exception event, with n)fjt
2192 2839 58 0 (ew)fjt
375 2779 1153 7 (values overwriting previous values, and coded as follows:)fjt
533 2686 25 0 (0)fjt
700 2686 229 0 (unspecified)fjt
533 2626 25 0 (1)fjt
700 2626 251 2 (peer IP error)fjt
533 2566 25 0 (2)fjt
700 2566 1283 8 (peer authentication failure \(peer.authentic bit was one now zero\))fjt
533 2506 25 0 (3)fjt
700 2506 1049 6 (peer unreachable \(peer.reach was nonzero now zero\))fjt
533 2446 25 0 (4)fjt
700 2446 999 6 (peer reachable \(peer.reach was zero now nonzero\))fjt
533 2386 25 0 (5)fjt
700 2386 979 7 (peer clock exception \(see peer clock status word\))fjt
533 2326 92 0 (6-15)fjt
700 2326 168 0 (reserved)fjt
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300 2235 125 0 (9.2.3.)fjt
(d)18 (r)30 (o)45 (W)13 ( )27 (s)29 (u)16 (t)26 (a)16 (t)32 (S)13 ( )27 (k)26 (c)30 (o)13 (l)35 (C)12 ( )0 18 425 2235 fet
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300 2142 1892 18 (There are two ways a reference clock can be attached to a NTP service host, as an dedicated dev)fjt
2192 2142 58 0 (ice)fjt
300 2083 1950 16 (managed by the operating system and as a synthetic peer managed by NTP \(see Section 3.4.4\). As)fjt
300 2025 1889 16 (in the read status command, the association identifier is used to identify which one, zero for )fjt
2189 2025 61 0 (the)fjt
300 1967 1950 16 (system clock and nonzero for a peer clock. Only one system clock is supported by the protocol,)fjt
300 1909 1950 16 (although many peer clocks can be supported. A system or peer clock status word appears in the)fjt
300 1850 1925 16 (status field of the response to a read clock variables or write clock variables command. This wor)fjt
2225 1850 25 0 (d)fjt
300 1792 1938 16 (can be considered an extension of the system status word or the peer status word as appropriate)fjt
2238 1792 13 0 (.)fjt
300 1734 985 9 (The format of the clock status word is as follows:)fjt
300 1639 1870 14 (Clock Status: This is an eight-bit integer indicating the current clock status, with values code)fjt
2170 1639 80 1 (d as)fjt
375 1579 163 0 (follows:)fjt
533 1487 25 0 (0)fjt
700 1487 643 3 (clock operating within nominals)fjt
533 1427 25 0 (1)fjt
700 1427 267 1 (reply timeout)fjt
533 1367 25 0 (2)fjt
700 1367 332 2 (bad reply format)fjt
533 1307 25 0 (3)fjt
700 1307 526 3 (hardware or software fault)fjt
533 1247 25 0 (4)fjt
700 1247 380 1 (propagation failure)fjt
533 1187 25 0 (5)fjt
700 1187 487 4 (bad date format or value)fjt
533 1127 25 0 (6)fjt
700 1127 492 4 (bad time format or value)fjt
533 1067 117 0 (7-255)fjt
700 1067 168 0 (reserved)fjt
300 975 1819 14 (Clock Event Code: This is an eight-bit integer identifying the latest clock exception event, wi)fjt
2119 975 132 1 (th new)fjt
375 915 1875 14 (values overwriting previous values. When a change to any nonzero value occurs in the radio)fjt
375 855 1875 18 (status field, the radio status field is copied to the clock event code field and a system or peer)fjt
375 795 966 6 (clock exception event is declared as appropriate.)fjt
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300 701 125 0 (9.2.4.)fjt
(d)19 (r)29 (o)45 (W)13 ( )27 (s)29 (u)16 (t)27 (a)15 (t)32 (S)13 ( )18 (r)30 (o)18 (r)18 (r)33 (E)12 ( )0 18 425 701 fet
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300 608 1782 19 (An error status word is returned in the status field of an error response as the result of invalid )fjt
2082 608 168 0 (message)fjt
300 549 1850 17 (format or contents. Its presence is indiated when the E \(error\) bit is set along with the respons)fjt
2150 549 100 1 (e \(R\))fjt
300 491 1394 12 (bit in the response. It consists of an eight-bit integer coded as follows:)fjt
533 399 25 0 (0)fjt
700 399 229 0 (unspecified)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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533 2839 25 0 (1)fjt
700 2839 424 1 (authentication failure)fjt
533 2779 25 0 (2)fjt
700 2779 655 4 (invalid message length or format)fjt
533 2719 25 0 (3)fjt
700 2719 295 1 (invalid opcode)fjt
533 2659 25 0 (4)fjt
700 2659 614 2 (unknown association identifier)fjt
533 2599 25 0 (5)fjt
700 2599 479 2 (unknown variable name)fjt
533 2539 25 0 (6)fjt
700 2539 431 2 (invalid variable value)fjt
533 2479 25 0 (7)fjt
700 2479 540 1 (administratively prohibited)fjt
533 2419 117 0 (8-255)fjt
700 2419 168 0 (reserved)fjt
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(s)30 (d)29 (n)27 (a)43 (m)44 (m)29 (o)35 (C)13 ( )0 9 383 2329 fet
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300 2240 1928 16 (Commands consist of the header and optional data field shown in Figure 6. When present, the dat)fjt
2228 2240 22 0 (a)fjt
300 2181 1188 10 (field contains a list of identifiers or assignments in the form)fjt
781 2090 988 0 (<identifier>[=<value>],<identifier>[=<value>],...)fjt
300 1999 1950 18 (where <identifier> is the ASCII name of a system or peer variable specified in Table 2 or Table 3)fjt
300 1940 1950 16 (and <value> is expressed as a decimal, hexadecimal or string constant in the syntax of the C)fjt
300 1882 1950 13 (programming language. Where no ambiguity exists, the "sys." or "peer." prefixes shown in Table)fjt
300 1824 1925 15 (2 or Table 4 can be suppressed. Whitespace \(ASCII nonprinting format effectors\) can be added t)fjt
2225 1824 25 0 (o)fjt
300 1766 1936 13 (improve readability for simple monitoring programs that do not reformat the data field. Interne)fjt
2236 1766 14 0 (t)fjt
300 1707 1796 15 (addresses are represented as four octets in the form [n.n.n.n], where n is in decimal notation)fjt
2096 1707 154 2 ( and the)fjt
300 1649 1816 10 (brackets are optional. Timestamps, including reference, originate, receive and transmit val)fjt
2116 1649 135 1 (ues, as)fjt
300 1591 1771 15 (well as the logical clock, are represented in units of seconds and fractions, preferably in hex)fjt
2071 1591 179 0 (adecimal)fjt
300 1533 1765 13 (notation, while delay, offset, dispersion and distance values are represented in units of mil)fjt
2065 1533 185 0 (liseconds)fjt
300 1474 1858 12 (and fractions, preferably in decimal notation. All other values are represented as-is, preferab)fjt
2158 1474 92 1 (ly in)fjt
300 1416 346 1 (decimal notation.)fjt
300 1325 1950 14 (Implementations may define variables other than those listed in Table 2 or Table 3. Called)fjt
300 1267 1878 13 (extramural variables, these are distinguished by the inclusion of some character type other t)fjt
2178 1267 72 0 (han)fjt
300 1208 1834 17 (alphanumeric or "." in the name. For those commands that return a list of assignments in the res)fjt
2134 1208 116 0 (ponse)fjt
300 1150 1826 16 (data field, if the command data field is empty, it is expected that all available variables defi)fjt
2126 1150 124 1 (ned in)fjt
300 1092 1950 18 (Table 3 or Table 4 will be included in the response. For the read commands, if the command data)fjt
300 1034 1864 14 (field is nonempty, an implementation may choose to process this field to individually select w)fjt
2164 1034 86 0 (hich)fjt
300 975 554 4 (variables are to be returned.)fjt
300 884 757 4 (Commands are interpreted as follows:)fjt
300 791 1950 16 (Read Status \(1\): The command data field is empty or contains a list of identifiers separated by)fjt
375 731 1875 13 (commas. The command operates in two ways depending on the value of the association)fjt
375 671 1749 13 (identifier. If this identifier is nonzero, the response includes the peer identifier and status)fjt
2124 671 127 1 ( word.)fjt
375 611 1806 14 (Optionally, the response data field may contain other information, such as described in the R)fjt
2181 611 69 0 (ead)fjt
375 551 1875 12 (Variables command. If the association identifier is zero, the response includes the system)fjt
375 491 1709 14 (identifier \(0\) and status word, while the data field contains a list of binary-coded pairs)fjt
885 399 780 3 (<association identifier> <status word>,)fjt
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1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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375 2839 846 5 (one for each currently defined association.)fjt
300 2694 1900 16 (Read Variables \(2\): The command data field is empty or contains a list of identifiers separated )fjt
2200 2694 50 0 (by)fjt
375 2634 1875 12 (commas. If the association identifier is nonzero, the response includes the requested peer)fjt
375 2574 1875 16 (identifier and status word, while the data field contains a list of peer variables and values as)fjt
375 2514 1836 15 (described above. If the association identifier is zero, the data field contains a list of syste)fjt
2211 2514 39 0 (m)fjt
375 2454 1875 16 (variables and values. If a peer has been selected as clock source, the response includes the peer)fjt
375 2394 1831 13 (identifier and status word; otherwise, the response includes the system identifier \(0\) and stat)fjt
2206 2394 44 0 (us)fjt
375 2334 128 1 (word. )fjt
300 2187 1873 15 (Write Variables \(3\): The command data field contains a list of assignments as described above. )fjt
2173 2187 78 0 (The)fjt
375 2127 1875 13 (variables are updated as indicated. The response is as described for the Read Variables)fjt
375 2067 209 0 (command.)fjt
300 1920 1851 16 (Read Clock Variables \(4\): The command data field is empty or contains a list of identifiers sepa)fjt
2151 1920 99 0 (rated)fjt
375 1860 1820 13 (by commas. The association identifier selects the system clock variables or peer clock variab)fjt
2195 1860 55 0 (les)fjt
375 1800 1875 15 (in the same way as in the Read Variables command. The response includes the requested clock)fjt
375 1740 1748 16 (identifier and status word and the data field contains a list of clock variables and values, inc)fjt
2123 1740 128 0 (luding)fjt
375 1680 1014 7 (the last timecode message received from the clock.)fjt
300 1532 1950 14 (Write Clock Variables \(5\): The command data field contains a list of assignments as described)fjt
375 1472 1875 15 (above. The clock variables are updated as indicated. The response is as described for the Read)fjt
375 1412 543 2 (Clock Variables command.)fjt
300 1265 1874 13 (Set Trap Address/Port \(6\): The command association identifier, status and data fields are igno)fjt
2174 1265 76 0 (red.)fjt
375 1205 1875 14 (The address and port number for subsequent trap messages are taken from the source address)fjt
375 1145 1758 15 (and port of the control message itself. The initial trap counter for trap response messages is)fjt
2133 1145 117 1 ( taken)fjt
375 1085 1875 13 (from the sequence field of the command. The response association identifier, status and data)fjt
375 1025 1875 11 (fields are not significant. Implementations should include sanity timeouts which prevent trap)fjt
375 965 1785 13 (transmissions if the monitoring program does not renew this information after a lengthy inte)fjt
2160 965 90 0 (rval.)fjt
300 818 1928 16 (Trap Response \(7\): This message is sent when a system, peer or clock exception event occurs. Th)fjt
2228 818 22 0 (e)fjt
375 758 1875 20 (opcode field is 7 and the R bit is set. The trap counter is incremented by one for each trap sent)fjt
375 698 1875 18 (and the sequence field set to that value. The trap message is sent using the IP address and port)fjt
375 638 1754 14 (fields established by the set trap address/port command. If a system trap the association ide)fjt
2129 638 121 0 (ntifier)fjt
375 578 1715 19 (field is set to zero and the status field contains the system status word. If a peer trap the ass)fjt
2090 578 160 0 (ociation)fjt
375 518 1861 16 (identifier field is set to that peer and the status field contains the peer status word. Optiona)fjt
2236 518 14 0 (l)fjt
375 458 1174 8 (ASCII-coded information can be included in the data field.)fjt
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1024 2979 504 2 (Network Time Protocol)fjt
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300 2841 69 0 (10.)fjt
(s)27 (e)29 (u)27 (s)27 (s)12 (I)12 ( )30 (n)29 (o)13 (i)15 (t)27 (a)27 (c)13 (i)15 (t)30 (n)26 (e)30 (h)15 (t)30 (u)35 (A)12 ( )12 (.)35 (C)12 ( )27 (x)13 (i)29 (d)30 (n)27 (e)29 (p)30 (p)35 (A)12 ( )0 34 369 2841 fet
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300 2751 1859 12 (NTP robustness requirements are similar to those of other multiple-peer distributed protocols )fjt
2159 2751 91 0 (used)fjt
300 2692 1887 12 (for network routing, management and file access. These include protection from faulty implem)fjt
2187 2692 47 0 (en)fjt
2234 2692 17 0 (-)fjt
300 2634 1816 12 (tations, improper operation and possibly malicious replay attacks with or without data modifi)fjt
2116 2634 134 0 (cation.)fjt
300 2576 1840 12 (These requirements are especially stringent with distributed protocols, since damage due to fa)fjt
2140 2576 110 0 (ilures)fjt
300 2518 1870 11 (can propagate quickly throughout the network, devastating archives, routes and monitoring syst)fjt
2170 2518 80 0 (ems)fjt
300 2459 1902 16 (and even bring down major portions of the network in the fashion of the classic Internet Worm.)fjt
300 2367 1900 12 (The access-control mechanism suggested in Section 3.5 responds to these requirements by limiti)fjt
2200 2367 50 0 (ng)fjt
300 2308 1950 14 (access to trusted peers. The various sanity checks resist most replay and spoofing attacks by)fjt
300 2250 1864 15 (discarding old duplicates and using the originate timestamp as a one-time pad, since it is unli)fjt
2164 2250 86 0 (kely)fjt
300 2192 1898 15 (that even a synchronized peer can predict future timestamps with the precision required on the ba)fjt
2198 2192 52 0 (sis)fjt
300 2134 1925 12 (of past observations alone. In addition, the protocol environment resists jamming attacks b)fjt
2225 2134 25 0 (y)fjt
300 2075 1855 11 (employing redundant time servers and diverse network paths. Resistance to stochastic disrupt)fjt
2155 2075 96 0 (ions,)fjt
300 2017 1796 13 (actual or manufactured, are minimized by careful design of the filtering and selection algo)fjt
2096 2017 140 0 (rithms.)fjt
300 1924 1811 12 (However, it is possible that a determined intruder can disrupt timekeeping operations betwee)fjt
2111 1924 139 1 (n peers)fjt
300 1866 1819 14 (by subtle modifications of NTP message data, such as falsifying header fields or certain times)fjt
2119 1866 131 0 (tamps.)fjt
300 1808 1887 14 (In cases where protection from even these types of attacks is required, a specifically enginee)fjt
2187 1808 64 0 (red)fjt
(l)22 (a)22 (c)13 (i)25 (p)25 (y)31 (T)25 ( )12 (.)25 (y)17 (r)22 (a)19 (s)19 (s)22 (e)22 (c)22 (e)25 (n)25 ( )20 (s)13 (i)25 ( )20 (s)22 (e)25 (u)25 (q)13 (i)26 (n)25 (h)22 (c)22 (e)14 (t)25 ( )23 (c)14 (i)25 (h)25 (p)22 (a)17 (r)25 (g)26 (o)14 (t)25 (p)25 (y)17 (r)22 (c)25 ( )25 (n)26 (o)25 ( )25 (d)22 (e)20 (s)22 (a)25 (b)25 ( )39 (m)20 (s)14 (i)25 (n)22 (a)25 (h)23 (c)22 (e)39 (m)25 ( )25 (n)25 (o)14 (i)14 (t)23 (a)22 (c)14 (i)14 (t)25 (n)22 (e)26 (h)14 (t)25 (u)22 (a)17 (-)22 (e)25 (g)23 (a)19 (s)20 (s)22 (e)39 (m)0 88 300 1750 fet
300 1691 1898 12 (mechanisms involve the use of cryptographic certificates, algorithms and key media, together w)fjt
2198 1691 53 0 (ith)fjt
300 1633 1950 12 (secure media databases and key-management protocols. Ongoing research efforts in this area are)fjt
300 1575 1900 13 (directed toward developing a standard methodology that can be used with many protocols, includi)fjt
2200 1575 50 0 (ng)fjt
300 1517 1815 13 (NTP. However, while it may eventually be the case that ubiquitous, widely applicable authent)fjt
2115 1517 135 0 (ication)fjt
300 1458 1950 12 (methodology may be adopted by the Internet community and effectively overtake the mechanism)fjt
300 1400 1886 13 (described here, it does not appear that specific standards and implementations will happen wit)fjt
2186 1400 64 0 (hin)fjt
300 1342 901 7 (the lifetime of this particular version of NTP.)fjt
300 1249 1950 12 (The NTP authentication mechanism described here is intended for interim use until specific)fjt
300 1191 1938 12 (standards and implementations operating at the network level or transport level are available)fjt
2238 1191 13 0 (.)fjt
300 1132 1861 15 (Support for this mechanism is not required in order to conform to the NTP specification itself.)fjt
2161 1132 89 1 ( The)fjt
300 1074 1950 12 (mechanism, which operates at the application level, is designed to protect against unauthorized)fjt
300 1016 1881 10 (message-stream modification and misrepresentation of source by insuring that unbroken, authe)fjt
2181 1016 53 0 (nti)fjt
2234 1016 17 0 (-)fjt
300 958 1878 13 (cated paths exist between a trusted, stratum-one server in a particular synchronization subnet )fjt
2178 958 72 0 (and)fjt
300 899 1878 15 (all other servers in that subnet. It employs a crypto-checksum, computed by the sender and chec)fjt
2178 899 72 0 (ked)fjt
300 841 1889 13 (by the receiver, together with a set of predistributed algorithms and cryptographic keys indexed)fjt
2189 841 62 1 ( by)fjt
300 783 1820 16 (a key identifier included in the message. However, there are no provisions in NTP itself to dis)fjt
2120 783 130 0 (tribute)fjt
300 725 1891 12 (or maintain the certificates, algorithms or keys. These quantities may occasionally be chang)fjt
2191 725 60 0 (ed,)fjt
300 666 1828 14 (which may result in inconsistent key information while rekeying is in progress. The nature o)fjt
2128 666 123 1 (f NTP)fjt
300 608 1740 15 (itself is quite tolerant to such disruptions, so no particular provisions are included to deal )fjt
2040 608 211 1 (with them.)fjt
300 457 1851 16 (The intent of the authentication mechanism is to provide a framework that can be used in conjun)fjt
2151 457 100 0 (ction)fjt
300 399 1950 12 (with selected mode combinations to build specific plans to manage clockworking communities and)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
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300 2841 1950 14 (implement policy as necessary. It can be selectively enabled or disabled on a per-peer basis)fjt
300 2782 1950 14 (\(peer.authenable and peer.authentic bits\). There is no specific plan proposed to manage the use of)fjt
300 2724 1909 13 (such schemes; although several possibilities are immediately obvious. In one scenario a group )fjt
2209 2724 42 0 (of)fjt
300 2666 1950 15 (time servers peers among themselves using symmetric modes and shares one secret key, say key 1,)fjt
300 2608 1950 13 (while another group of servers peers among themselves using symmetric modes and shares another)fjt
300 2549 1950 18 (secret key, say key 2. Now, assume by policy it is decided that selected servers in group 1 can)fjt
300 2491 1950 16 (provide synchronization to group 2, but not the other way around. The selected servers in group 1)fjt
300 2433 1936 16 (are given key 2, but operated only in server mode, so cannot accept synchronization from group 2)fjt
2236 2433 14 0 (;)fjt
300 2375 1928 13 (however, group 2 has authenticated access to group-1 servers. Many other scenarios are possibl)fjt
2228 2375 22 0 (e)fjt
300 2316 934 6 (with suitable combinations of modes and keys.)fjt
300 2229 1886 13 (A packet format and crypto-checksum procedure appropriate for NTP is specified in the follow)fjt
2186 2229 64 0 (ing)fjt
300 2171 1934 12 (sections. The cryptographic information is carried in an authenticator which follows the \(un)fjt
2234 2171 17 0 (-)fjt
300 2113 1950 11 (modified\) NTP header fields. The crypto-checksum procedure uses the Data Encryption Standard)fjt
300 2054 1876 14 (\(DES\) [2]; however, only the DES encryption algorithm is used and the decryption algorithm is)fjt
2176 2054 74 1 ( not)fjt
300 1996 1839 11 (necessary. This feature is specifically targeted toward governmental sensitivities on the exp)fjt
2139 1996 111 1 (ort of)fjt
300 1938 1950 12 (cryptographic technology, since the DES decryption algorithm need not be included in NTP)fjt
300 1880 1843 14 (software distributions and thus cannot be extracted and used in other applications to avoid me)fjt
2143 1880 108 0 (ssage)fjt
300 1821 309 1 (data disclosure.)fjt
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300 1733 111 0 (10.1.)fjt
(m)26 (s)13 (i)30 (n)26 (a)30 (h)27 (c)26 (e)41 (M)12 ( )29 (n)30 (o)13 (i)15 (t)27 (a)27 (c)12 (i)16 (t)29 (n)27 (e)30 (h)15 (t)30 (u)35 (A)12 ( )31 (P)30 (T)35 (N)12 ( )0 29 411 1733 fet
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300 1646 1831 14 (When it is created and possibly at other times, each association is allocated variables ident)fjt
2131 1646 119 0 (ifying)fjt
300 1587 1873 11 (the certificate authority, encryption algorithm, cryptographic key and possibly other data. )fjt
2173 1587 78 0 (The)fjt
300 1529 1757 14 (specific procedures to allocate and initialize these variables are beyond the scope of this )fjt
2057 1529 176 0 (specifica)fjt
2234 1529 17 0 (-)fjt
300 1471 1859 15 (tion, as are the association of the identifiers and keys and the management and distribution o)fjt
2159 1471 91 1 (f the)fjt
300 1413 1950 14 (keys themselves. For example and consistency with the conventions of Section 3.3, a set of)fjt
300 1354 1324 8 (appropriate peer and packet variables might include the following:)fjt
300 1265 1864 11 (Key Identifier \(sys.keyid, peer.keyid, pkt.keyid\): This is an integer identifying the cryptogra)fjt
2164 1265 86 0 (phic)fjt
375 1205 1875 12 (key used to generate the message-authentication code as described below. The system variable)fjt
375 1145 1859 13 (sys.keyid is used for active associations. The peer.keyid variable is initialized at zero \(un)fjt
2234 1145 17 0 (-)fjt
375 1085 1767 12 (specified\) when the association is mobilized. For purposes of authentication an unassigned )fjt
2142 1085 108 0 (value)fjt
375 1025 700 4 (is interpreted as zero \(unspecified\).)fjt
300 935 1928 17 (Cryptographic Keys \(sys.key\): These are a set of 64-bit DES keys. Each key is constructed as in th)fjt
2228 935 22 0 (e)fjt
375 875 1875 13 (Berkeley Unix distributions, which consists of eight octets, where the seven low-order bits of)fjt
375 815 1875 15 (each octet correspond to the DES bits 1-7 and the high-order bit corresponds to the DES)fjt
375 755 1812 14 (odd-parity bit 8. By convention, the unspecified key 0 \(zero\), consisting of eight odd-parity z)fjt
2187 755 64 0 (ero)fjt
375 695 1875 13 (octets, is used for testing and presumed known throughout the NTP community. The remaining)fjt
375 635 1220 9 (keys are distributed using methods outside the scope of NTP.)fjt
300 545 1947 10 (Crypto-Checksum \(pkt.check\): This is a crypto-checksum computed by the encryption procedure.)fjt
300 457 1850 13 (The authenticator field consists of two subfields, one consisting of the pkt.keyid variable an)fjt
2150 457 100 1 (d the)fjt
300 399 1950 14 (other the pkt.check variable computed by the encrypt procedure, which is called by the transmit)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
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300 2439 1889 15 (procedure described in Section 3.4.1, and by the decrypt procedure, which is called by the rece)fjt
2189 2439 61 0 (ive)fjt
300 2381 1911 15 (procedure described in Sectio 3.4.2. Its presence is revealed by the fact the total datagram leng)fjt
2211 2381 39 0 (th)fjt
300 2322 1950 15 (according to the UDP header is longer than the NTP message length, which includes the header)fjt
300 2264 1950 14 (plus the data field, if present. For authentication purposes, the NTP message is zero-padded if)fjt
300 2206 1843 16 (necessary to a 64-bit boundary, although the padding bits are not considered part of the NTP me)fjt
2143 2206 108 0 (ssage)fjt
300 2148 987 7 (itself. The authenticator format shown in Figure 8)fjt
1287 2148 963 9 ( has 96 bits, including a 32-bit key identifier and)fjt
300 2089 1851 12 (64-bit crypto-checksum, and is aligned on a 32-bit boundary for efficient computation. Addit)fjt
2151 2089 100 0 (ional)fjt
300 2031 1833 10 (information required in some implementations, such as certificate authority and encryption)fjt
2133 2031 101 1 ( algo)fjt
2234 2031 17 0 (-)fjt
300 1973 1950 16 (rithm, can be inserted between the \(padded\) NTP message and the key identifier, as long as the)fjt
300 1915 1776 12 (alignment conditions are met. Like the authenticator itself, this information is not includ)fjt
2076 1915 175 2 (ed in the)fjt
300 1856 1931 13 (crupto-checksum. Use of these data are beyond the scope of this specification. These convention)fjt
2231 1856 19 0 (s)fjt
300 1798 1445 11 (may be changed in future as the result of other standardization activities.)fjt
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300 1710 111 0 (10.2.)fjt
(s)27 (e)18 (r)30 (u)29 (d)27 (e)27 (c)29 (o)19 (r)32 (P)12 ( )29 (n)30 (o)13 (i)15 (t)27 (a)27 (c)12 (i)16 (t)29 (n)27 (e)30 (h)15 (t)30 (u)35 (A)12 ( )31 (P)30 (T)35 (N)12 ( )0 30 411 1710 fet
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300 1623 1864 12 (When authentication is implemented there are two additional procedures added to those descr)fjt
2164 1623 86 0 (ibed)fjt
300 1564 1950 12 (in Section 3.4. One of these \(encrypt\) constructs the crypto-checksum in transmitted messages,)fjt
300 1506 1936 14 (while the other \(decrypt\) checks this quantity in received messages. The procedures use a varian)fjt
2236 1506 14 0 (t)fjt
300 1448 1862 15 (of the cipher-block chaining method described in [10] as applied to DES. In principal, the proce)fjt
2162 1448 89 0 (dure)fjt
300 1390 1938 14 (is independent of DES and requires only that the encryption algorithm operate on 64-bit blocks)fjt
2238 1390 13 0 (.)fjt
300 1331 1950 14 (While the NTP authentication mechanism specifies the use of DES, other algorithms could be used)fjt
300 1273 433 2 (by prior arrangement.)fjt
300 1186 1854 12 (For ordinary NTP messages the encryption procedure operates as follows. If authentication i)fjt
2154 1186 97 1 (s not)fjt
300 1127 1925 12 (enabled \(peer.authenable set to zero\), the procedure simply exits. Otherwise, a 64-bit temporar)fjt
2225 1127 25 0 (y)fjt
300 1069 1851 16 (variable is initialized to zero. For each of the 64-bit NTP header and data words not includin)fjt
2151 1069 99 1 (g the)fjt
300 1011 1786 12 (authenticator or additional information and proceeding from the beginning of the header, th)fjt
2086 1011 164 1 (e header)fjt
300 953 1950 14 (word is XORed with the temporary variable and the variable then encrypted using the DES)fjt
300 894 1815 17 (algorithm. If the association is active \(modes 1, 3, 5\) the key is determined by the system v)fjt
2115 894 135 0 (ariable)fjt
300 836 1928 16 (sys.keyid. If the association is passive \(modes 2, 4\) the key is determined by the peer variabl)fjt
2228 836 22 0 (e)fjt
(e)25 (h)14 (t)25 ( )12 (,)25 (y)14 (l)14 (l)22 (a)25 (n)13 (i)28 (F)25 ( )13 (.)22 (e)19 (s)14 (i)36 (w)16 (r)22 (e)25 (h)14 (t)25 (o)25 ( )16 (\))25 (o)17 (r)22 (e)22 (z)16 (\()25 ( )25 (y)22 (e)25 (k)25 ( )14 (t)14 (l)25 (u)22 (a)16 (f)22 (e)25 (d)25 ( )22 (e)25 (h)14 (t)25 ( )25 (d)25 (n)22 (a)25 ( )22 (e)25 (n)25 (o)25 ( )25 (o)14 (t)25 ( )14 (t)22 (e)19 (s)25 ( )19 (s)14 (i)25 ( )22 (c)14 (i)13 (t)25 (n)22 (e)25 (h)14 (t)25 (u)22 (a)13 (.)16 (r)22 (e)22 (e)25 (p)25 ( )17 (f)13 (i)25 ( )25 (d)14 (i)25 (y)22 (e)25 (k)13 (.)16 (r)22 (e)23 (e)25 (p)0 93 300 778 fet
300 720 1844 13 (authenticator is constructed using the chosen key for pkt.keyid and temporary variable for pkt.c)fjt
2144 720 107 0 (heck.)fjt
300 632 1837 14 (For ordinary messages the decryption procedure operates as follows. If the peer is not confi)fjt
2137 632 114 0 (gured)fjt
300 574 1863 15 (\(peer.config bit set to zero\) and the message data includes the authenticator, which is placed a)fjt
2163 574 87 1 (t the)fjt
300 516 1865 18 (end of the NTP message itself, the peer.authenable bit is set to one; otherwise, it is set to zer)fjt
2165 516 85 1 (o. If)fjt
300 457 1950 16 (peer.config is set to one, no change to peer.authenable is made. If peer.authenable is set to zero)fjt
300 399 1903 15 (following this step, the procedure simply exits. Then, if the message data does not include t)fjt
2203 399 47 0 (he)fjt
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1057 2677 461 1 (Crypto-Checksum \(64\))fjt
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850 2846 25 0 (0)fjt
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1713 2846 50 0 (31)fjt
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1075 2846 25 0 (8)fjt
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1288 2846 50 0 (16)fjt
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1099 2771 376 2 (Key Identifier \(32\))fjt
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964 2543 622 3 (Figure 8. Authenticator Format)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
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300 2841 1744 13 (authenticator fields, the peer.authentic bit is set to zero and the procedure exits. Otherwise,)fjt
2044 2841 207 2 ( the packet)fjt
300 2782 1925 13 (variable pkt.keyid is copied to the peer variable peer.keyid and the crypto-checksum is compute)fjt
2225 2782 25 0 (d)fjt
300 2724 1886 16 (using that variable. The peer.authentic bit is set to one if peer.keyid is nonzero and the checks)fjt
2186 2724 64 0 (um)fjt
300 2666 1560 13 (matches the pkt.check field following this step; otherwise the bit is set to zero.)fjt
300 2579 1950 16 (For NTP control messages the peer variables are not used. If a command message is received with)fjt
300 2520 1815 14 (an authenticator field, the crypto-checksum is computed as in the decrypt procedure and the re)fjt
2115 2520 136 0 (sponse)fjt
300 2462 1950 13 (message includes the authenticator field as computed by the encrypt procedure. If the received)fjt
300 2404 1812 17 (authenticator is correct the key for the response is the same as in the command; otherwise, the )fjt
2112 2404 138 0 (default)fjt
300 2346 1950 17 (key \(zero\) is used. Commands causing a change to the peer data base, such as the write variables)fjt
300 2287 1950 11 (and set trap address/port commands, must be correctly authenticated; however, the remaining)fjt
300 2229 1761 11 (commands are normally not authenticated in order to minimize the encryption overhead.)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 2841 69 0 (11.)fjt
(.)27 (s)29 (n)30 (o)12 (i)27 (s)18 (r)27 (e)30 (V)12 ( )27 (s)29 (u)30 (o)12 (i)27 (v)27 (e)18 (r)32 (P)13 ( )43 (m)30 (o)18 (r)15 (f)13 ( )27 (s)26 (e)27 (c)30 (n)26 (e)19 (r)26 (e)16 (f)15 (f)13 (i)35 (D)13 ( )12 (.)35 (D)13 ( )27 (x)12 (i)30 (d)29 (n)27 (e)30 (p)29 (p)35 (A)13 ( )0 48 369 2841 fet
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300 2749 1950 16 (The original NTP, later called NTP Version 0, was described in RFC-958 [30], while the most recent)fjt
300 2691 1950 15 (prior NTP Version 1 was described in RFC-1059 [42]. The Version-2 description has been split into)fjt
300 2633 1865 12 (two documents, this one defining the architecture and specifying the protocol and algorithms,)fjt
2165 2633 85 1 ( and)fjt
300 2575 1950 11 (another [44] describing the service model, algorithmic analysis and operating experience. In)fjt
300 2516 1950 12 (previous versions [30], [42] these two objectives were combined in one document. Differences)fjt
300 2458 1950 15 (between NTP Version 2 and previous versions are described in this Appendix. Due to known bugs)fjt
300 2400 1819 11 (in very old implementations, continued support for Version-0 implementations is not recomm)fjt
2119 2400 132 0 (ended.)fjt
300 2342 1898 12 (It is recommended that new implementations follow the guidelines below when interoperating w)fjt
2198 2342 53 0 (ith)fjt
300 2283 556 1 (Version-1 implementations.)fjt
300 2186 38 0 (1.)fjt
375 2186 1875 11 (Version 1 supports no modes other than symmetric-active and symmetric-passive, which are)fjt
375 2128 1875 13 (determined by inspecting the port-number fields of the UDP packet header as described in)fjt
375 2070 1853 17 (Section 3.3 above. The low-order three bits of the first octet, specified as zero in Version 1, ar)fjt
2228 2070 22 0 (e)fjt
375 2011 1834 12 (used for the mode field in Version 2. Version-2 implementations interoperating with Version)fjt
2209 2011 42 0 (-1)fjt
375 1953 1875 15 (implementations should operate in a passive mode only and use the value one in the version)fjt
375 1895 1790 12 (number \(pkt.version\) field and zero in the mode \(pkt.mode\) field in transmitted messages.)fjt
300 1798 38 0 (2.)fjt
375 1798 1875 14 (Version 1 does not support the NTP control message described in Appendix B. Certain old)fjt
375 1739 682 6 (versions of the Unix NTP daemon )fjt
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1057 1739 89 0 (ntpd)fjt
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1146 1739 1104 9 ( use the high-order bits of the stratum field \(pkt.stratum\))fjt
375 1681 1875 14 (for control and monitoring purposes. While these bits are never set during normal Version-1 or)fjt
375 1623 1875 12 (Version-2 operations, new implementations may use the NTP reserved mode 6 described in)fjt
375 1565 1875 14 (Appendix B and/or private reserved mode 7 for special purposes, such as remote control and)fjt
375 1506 1799 16 (monitoring, and in such cases the format of the packet following the first octet can be arbitr)fjt
2174 1506 76 0 (ary.)fjt
375 1448 1875 11 (While there is no guarantee that different implementations can interoperate using private)fjt
375 1390 1801 13 (reserved mode 7, it is recommended that vanilla ASCII format be used whenever possible.)fjt
300 1292 38 0 (3.)fjt
375 1292 1859 12 (Version 1 does not support authentication. The key identifiers, cryptographic keys and proce)fjt
2234 1292 17 0 (-)fjt
375 1234 1875 14 (dures described in Appendix C are new to Version 2, along with the corresponding variables,)fjt
375 1176 1875 13 (procedures and authenticator fields. In the NTP message described in Appendix A and NTP)fjt
375 1118 1875 14 (control message described in Appendix B the format and contents of the header fields are)fjt
375 1059 1875 11 (independent of the authentication mechanism and the authenticator itself follows the header)fjt
375 1001 1216 8 (fields, so that previous versions will ignore the authenticator.)fjt
300 904 38 0 (4.)fjt
375 904 1875 13 (In Version 1 the synchronizing dispersion \(pkt.dispersion\) field of the NTP header was called)fjt
375 846 1792 13 (the estimated drift rate, but not used in the protocol or timekeeping procedures. Implementat)fjt
2167 846 83 0 (ions)fjt
375 788 1811 15 (of the Version-1 protocol typically set this field to the current value of the Drift Compensat)fjt
2186 788 64 0 (ion)fjt
375 729 1817 15 (Register, which is a signed quantity. In a Version 2 implementation apparent large values in t)fjt
2192 729 58 0 (his)fjt
375 671 1859 11 (field may affect the order considered in the clock-selection procedure. Version-2 implementa)fjt
2234 671 17 0 (-)fjt
375 613 1820 11 (tions interoperating with older implementations should assume this field is zero, regardless)fjt
2195 613 55 1 ( of)fjt
375 555 369 2 (its actual contents.)fjt
300 457 38 0 (5.)fjt
(n)26 (u)25 ( )25 (o)14 (t)25 ( )22 (e)25 (u)26 (d)25 ( )19 (s)25 (n)26 (o)14 (i)14 (t)25 (p)25 (u)17 (r)19 (s)14 (i)26 (d)25 ( )25 (d)14 (i)25 (o)25 (v)23 (a)25 ( )25 (o)14 (t)25 ( )25 (d)22 (e)26 (n)25 (g)14 (i)19 (s)23 (e)25 (d)25 ( )19 (s)26 (k)22 (c)23 (e)25 (h)23 (c)25 ( )25 (y)15 (t)14 (i)25 (n)23 (a)20 (s)25 ( )14 (l)22 (a)17 (r)23 (e)25 (v)23 (e)20 (s)25 ( )19 (s)23 (e)14 (t)23 (a)17 (r)25 (o)26 (p)17 (r)25 (o)23 (c)25 (n)14 (i)25 ( )26 (2)25 ( )25 (n)26 (o)14 (i)20 (s)17 (r)22 (e)37 (V)0 84 375 457 fet
(-)0 1 2234 457 fet
375 399 1839 12 (synchronized, duplicate or bogus timestamp information. The leap-indicator bits are set to sho)fjt
2214 399 36 0 (w)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 279 100 0 (Mills)fjt
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375 2841 1859 17 (the unsynchronized state if updates are not received from a reference source for a full day or i)fjt
2234 2841 17 0 (f)fjt
375 2782 1875 13 (the reference source has not received updates for a full day. Some Version-1 implementations)fjt
375 2724 1693 10 (could claim valid synchronization indefinitely following loss of the reference source.)fjt
300 2635 38 0 (6.)fjt
375 2635 1792 13 (The clock-selection procedure of Version 2 is considerably refined as the result of accumul)fjt
2167 2635 83 0 (ated)fjt
375 2577 1875 10 (experience with the Version-1 implementation. Additional sanity checks are included for)fjt
375 2519 1841 16 (authentication, range bounds and to avoid use of very old data. The candidate list is sorted twic)fjt
2216 2519 35 0 (e,)fjt
375 2460 1836 14 (once to select a relatively few robust candidates from a potentially large population of unru)fjt
2211 2460 39 0 (ly)fjt
375 2402 1875 16 (peers and again to order the resulting list by measurement quality. As in Version 1, The final)fjt
375 2344 1691 11 (selection procedure repeatedly casts out outlyers on the basis of weighted dispersion.)fjt
300 2255 38 0 (7.)fjt
375 2255 1875 14 (The local-clock procedure of Version 2 is considerably improved over Version 1 as the result)fjt
375 2196 1875 14 (of analysis, simulation and experience. Checks have been added to warn that the oscillator has)fjt
375 2138 1875 14 (gone too long without update from a reference source. The Compliance Register has been added)fjt
375 2080 1875 14 (to improve frequency stability to the order of a millisecond per day. The various parameters)fjt
375 2022 1875 13 (were retuned for optimum loop stability using measured data over typical Internet paths and)fjt
375 1963 680 3 (with typical local-clock hardware.)fjt
300 1874 38 0 (8.)fjt
375 1874 1875 13 (Problems in the timekeeping calculations of Version 1 with high-speed LANs were found and)fjt
375 1816 1875 14 (corrected. These were caused by jitter due to small differences in clock rates and different)fjt
375 1758 1861 12 (precisions between the peers. Subtle bugs in the Version-1 reachability and polling-rate contro)fjt
2236 1758 14 0 (l)fjt
375 1699 1875 13 (were found and corrected. The peer.valid and sys.hold variables were added to avoid instabilities)fjt
375 1641 1875 13 (when the reference source changes rapidly due to large dispersive delays under conditions of)fjt
375 1583 1875 10 (severe network congestion. The peer.config, peer.authenable and peer.authentic bits were added)fjt
375 1525 1068 6 (to control special features and simplify configuration.)fjt
300 1348 466 1 (Security considerations)fjt
375 1288 897 7 (see Section 3.5 and Section 10 \(Appendix C\))fjt
300 1198 339 1 (Author's address)fjt
375 1138 295 2 (David L. Mills)fjt
375 1078 693 2 (Electrical Engineering Department)fjt
375 1018 467 2 (University of Delaware)fjt
375 958 387 2 (Newark, DE 19716)fjt
375 898 449 2 (Phone \(302\) 451-8247)fjt
375 838 452 1 (EMail mills@udel.edu)fjt
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1882 2979 369 1 (September 1989)fjt
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